WASHINGTON (AP) - Here are key dates in the House impeachment inquiry into United States President Donald Trump:

APRIL 21, 2019: President Trump speaks with then President-elect Volodymyr Zelensky to congratulate him on his election victory. The White House says in a readout of the call that the Mr Trump underscored unwavering support for Ukraine's sovereignty and territorial integrity and expressed his commitment to work with Mr Zelensky to implement reforms that "strengthen democracy, increase prosperity, and root out corruption".

A rough transcript of the call, released in November, bears little resemblance to that description. The word "corruption" is not mentioned in the rough transcript, nor is there any reference territorial integrity.

JULY 3: Lt Col Alexander Vindman, a national security official working at the White House, becomes aware that military aid to Ukraine has been held up. He testified later that he received a notice from the State Department. "That's when I was concretely made aware of the fact there was a hold placed," he said in testimony to lawmakers.

JULY 10: A meeting at the White House with Ukrainian officials is cut short when Mr Gordon Sondland, the US ambassador to the European Union, says he has an agreement with acting White House chief of staff Mick Mulvaney that Ukraine's President would get a meeting with Mr Trump if Ukraine agreed to open investigations. National Security Adviser John Bolton "stiffened" and ended the meeting, later telling colleague Fiona Hill to report it to the National Security Council's lawyer, she testified.

"I am not part of whatever drug deal Sondland and Mulvaney are cooking up on this," Ms Hill says Mr Bolton told her.

JULY 18: In a secure call with national security officials, a staff member of the White House Office of Management and Budget announces there's a freeze on Ukraine aid until further notice, based on a presidential order to the budget office.

JULY 24: Special counsel Robert Mueller testifies before Congress that Mr Trump was not cleared of obstructing justice, nor was he completely exonerated in the Russia probe, as Mr Trump has contended. Mr Mueller issues a stark warning about the dangers of Russian interference in American elections.

JULY 25: Mr Trump has a second phone call with Mr Zelensky, now president, during which he solicits Mr Zelensky's help in gathering potentially damaging information about his principal Democratic rival, former vice-president Joe Biden.

That night, a staff member at the White House Office of Management and Budget signs a document that officially puts military aid for Ukraine on hold.

BETWEEN JULY 25 AND AUG 12: An unidentified CIA officer files a complaint with the agency alleging misconduct during the President's July 25 call, according to a person familiar with the matter.

JULY 26: US Special Representative for Ukraine Negotiations Kurt Volker visits Kiev and meets Mr Zelensky and various Ukrainian political officials, according to a whistle-blower complaint addressed to Congress and delivered to the intelligence community's inspector general. Mr Sondland also participates, and the two reportedly provide advice on how to "navigate" the President's demands.

Also, Mr Trump speaks by phone with Mr Sondland while the ambassador was in a Kiev restaurant. Mr William Taylor, the acting US ambassador to Ukraine, later tells lawmakers that one of his staffers overheard parts of the conversation.

Mr Sondland tells the President that the Ukrainians are ready to move forward, and after the call, one of Mr Taylor's staffers asks Mr Sondland what Mr Trump thought about Ukraine. "Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Mr Taylor later testified.

ON OR ABOUT AUG 2: Trump lawyer Rudy Giuliani reportedly travels to Madrid to meet one of Mr Zelensky's advisers, Mr Andriy Yermak, according to a whistle-blower complaint.

AUG 12: A whistle-blower complaint bearing this date and intended for Congress states: "In the course of my official duties, I have received information from multiple US Government officials that the President of the United States is using the power of his office to solicit interference from a foreign country in the 2020 US election."

The complaint is addressed to Senator Richard Burr, chairman of the Senate Select Committee on Intelligence, and Representative Adam Schiff, chairman of the House Permanent Select Committee on Intelligence. It does not reach them until Sept 25.

AUG 14: The whistle blower's original complaint to the CIA is brought up by Ms Courtney Simmons Elwood, general counsel for the CIA, during a call involving US national security officials, including Mr John Eisenberg, a White House lawyer, and Mr John Demers, who leads the Justice Department's national security division, according to a person familiar with the matter.

AUG 15: Mr Demers goes to the White House to review materials associated with the Zelensky call.

AUG 26: Mr Michael Atkinson, the inspector general for the intelligence community, sends a letter to the acting director of national intelligence informing him that the IG's office has received a complaint addressed to Congress of "urgent concern" about a call between Mr Trump and Mr Zelensky. The inspector general says he believes the conversation could have amounted to a federal campaign finance crime.

AUG 28: Politico reports that the military aid to Ukraine is on hold, setting off a scramble among diplomats in Ukraine and the United States.

SEPT 3: The Justice Department's office of legal counsel sends a memorandum to a lawyer at the Office of the Director of National Intelligence, finding that the "alleged misconduct does not involve any member of the intelligence community" and concludes that the Aug 12 complaint does not meet the statutory requirement as a matter of "urgent concern" that would require it to be forwarded to Congress.

SEPT 9: The inspector general for the intelligence community sends a letter to Mr Schiff and Mr Devin Nunes, ranking member of the House Intelligence Committee, about the whistle blower's complaint, saying that withholding it "does not appear to be consistent with past practice" because the acting DNI, Mr Joseph Maguire, is not permitting its release to Congress. Mr Atkinson, the inspector general, said in the letter that he is working with Mr Maguire to try to bring the whistle blower's concerns to Congress.

SEPT 11: The White House informs lawmakers that it is releasing US$250 million (S$338 million) in military aid to Ukraine. The US began providing military aid to the government of Ukraine shortly after Russia illegally annexed Ukraine's Crimean Peninsula in 2014. The US$250 million in funding had been delayed because "the President has been consulting with his national security leadership team to determine the best use of Ukraine security assistance funds to achieve US national security interests", Office of Management and Budget staff wrote in an e-mail to House Appropriations Committee staff aides.

SEPT 19: Mr Atkinson testifies behind closed doors to members of the House Intelligence Committee about the whistle blower's complaint. He does not give details about the substance of the complaint.

Also, the President begins responding to published reports about his phone call, tweeting that he understands many people from various US agencies listened in. "Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call. I would only do what is right anyway, and only do good for the USA!" he said.

SEPT 24: House Speaker Nancy Pelosi announces that the House is moving forward with an official impeachment inquiry, saying: "No one is above the law."

SEPT 25: The White House releases a rough transcript of the President's July 25 call with Mr Zelensky, confirming that Mr Trump has pushed Ukraine's leader to work with Mr Giuliani and Attorney General William Barr to investigate Mr Biden and his son. The Justice Department releases a statement saying prosecutors reviewed the inspector general's referral about a possible campaign finance violation and determined that no crime was committed.

The whistle blower's Aug 12 complaint is also transmitted to Congress.

SEPT 26: The House Intelligence Committee releases a redacted version of the whistle-blower complaint, now at the centre of Democrats' impeachment probe. The committee also receives testimony from Mr Maguire, who says the whistle blower "did the right thing" by coming forward to report concerns over the White House's handling of the call between Mr Trump and Ukraine's leader.

OCT 31: The Democratic-controlled House votes 232-196 to pass a resolution setting procedures for the impeachment inquiry as Democrats try to counter the Trump administration's criticism of the probe. Two Democrats voted against the resolution.

Career diplomat William Taylor, the charge d'affaires in Kiev, offered surprising new testimony that Mr Trump was overheard on a telephone call asking about "the investigations" of Democrats he wanted Ukraine to pursue. Former US Ambassador to Ukraine Marie Yovanovitch recounted how threatened she felt to learn that Mr Trump had promised Mr Zelensky that she was "going to go through some things". Mr Trump tweeted fresh criticism of Ms Yovanovitch as she testified. Mr Sondland testified that a "quid pro quo" existed and that "everyone was in the loop. It was no secret". And Ms Hill, Mr Trump's former Russia adviser, said Mr Sondland was running a "domestic political errand" and she had warned him it would "blow up".

NOV 15: Mr Trump releases a rough transcript of the congratulatory phone call he had with Mr Zelensky on April 21 and holds it out as evidence that he had done nothing wrong.

Nov 17: Ms Pelosi, in an interview broadcast by CBS News, invites Mr Trump to testify in front of impeachment investigators. The following day, Mr Trump tweets that "I like the idea & will, in order to get Congress focused again, strongly consider it!"

Nov 20: Mr Pence says he has no recollection of a conversation Mr Sondland described having with him about a link between military aid for Ukraine and investigations sought by Mr Trump. Mr Sondland had testified that he spoke with Mr Pence before a Sept 1 meeting with Ukrainian officials and expressed "concerns that the delay in aid had become tied to the issue of investigations". Mr Pence tells a Wisconsin TV station that he did not recall the conversation. A top Pence aide had said the call "never happened".

NOV 23: "Frankly, I want a trial" in the Senate, Mr Trump declares in a telephone interview on Fox & Friends.

DEC 3: A 300-page report prepared by Democrats on the House Intelligence Committee finds "serious misconduct" by the President.

DEC 4: The House Judiciary Committee holds its first hearing in the impeachment inquiry while Mr Trump attends a Nato conference in London.

DEC 5: Ms Pelosi announces that she has asked the relevant House committee chairs to begin drawing up articles of impeachment against Mr Trump, saying his actions left them "no choice" but to act swiftly. In response, Mr Trump tweets that Democrats have "gone crazy".

DEC 9: In a hearing before the House Judiciary Committee, Democrats outline their case against Mr Trump by saying his push to get Ukraine to investigate Mr Biden while withholding US military aid ran counter to US policy and benefited Russia as well as himself. Republicans reject the assertions.

DEC 10: Ms Pelosi and the relevant House committee chairs announce two articles of impeachment against Mr Trump, for abuse of power and for obstruction of justice, over charges that he threatened the integrity of US elections and endangered national security in his dealings with Ukraine. Mr Trump responds by tweeting "WITCH HUNT!"

DEC 13: House Judiciary Committee approves two articles of impeachment against Mr Trump, sending them to the full House.

DEC 18: House debates articles of impeachment.