WASHINGTON (REUTERS) - United States Vice-President Mike Pence and President Donald Trump's personal lawyer Rudy Giuliani said on Tuesday (Oct 15) that they will not cooperate with a House of Representatives impeachment inquiry into Mr Trump's efforts to pressure Ukraine to investigate a political rival.

The decision to defy congressional subpoenas opens the two men to possible contempt of Congress charges and illustrates Republican Mr Trump's determination to stonewall the Democratic-led impeachment effort, which threatens to consume his presidency.

The refusal to cooperate could be considered as evidence of obstruction of justice, which would substantiate suspicions about Mr Trump's misconduct, according to an official working on the inquiry who communicated on condition of anonymity.

But other US government officials have not been as reluctant to cooperate.

Lawmakers were hearing closed-door testimony on Tuesday from a senior US diplomat, Mr George Kent, who is one of several officials involved in the Ukrainian matter who have complied with congressional subpoenas.

House Democrats are focusing on Mr Trump's request to Ukrainian President Volodymyr Zelensky in a July 25 phone call to look into unsubstantiated allegations about Mr Joe Biden, the former vice-president and a leading contender to become the Democratic nominee to run against Mr Trump in the November 2020 US presidential election.

Mr Kent, who has spent much of his career fighting corruption in Ukraine and elsewhere, is the second career diplomat to testify as part of the probe after being subpoenaed. The White House and State Department had ordered them not to appear.

It was unclear what Mr Kent, a deputy assistant secretary of state responsible for US policy toward six former Soviet republics including Ukraine, was telling lawmakers.

According to the New York Times, he raised concerns with colleagues as far back as March about Mr Giuliani pressuring Ukraine to pursue investigations into Mr Trump's political rivals.

Ms Marie Yovanovitch, the former US ambassador to Ukraine, on Friday accused the Trump administration in testimony of recalling her in May based on false claims.

Ms Fiona Hill, Trump's former Russia adviser, told lawmakers on Monday that she and her then-boss, former national security adviser John Bolton, were alarmed this summer by efforts to force Ukraine to investigate Mr Biden and other rivals and advised her to notify a National Security Council lawyer, according to a source familiar with her testimony.

Ms Hill told lawmakers that Mr Bolton characterised Mr Giuliani as "a hand grenade who is going to blow everybody up", according to two sources familiar with her testimony. A Bolton spokesman said he would have no comment on the testimony.

Mr Gordon Sondland, a Trump political donor appointed as the US ambassador to the European Union who was involved in the Ukraine discussions, is due to testify later in the week in response to a congressional subpoena.

Mr Giuliani had faced a Tuesday deadline to produce the requested documents. His lawyer, Mr Jon Sale, said the documents were protected by legal principles that protect attorney-client communications and presidential communications.

"This appears to be an unconstitutional, baseless, and illegitimate 'impeachment inquiry,'" Mr Sale wrote, echoing language the White House used last week.

Mr Pence's lawyer, Mr Matthew E. Morgan, also cited the White House's response, saying the House had not voted to authorise the inquiry. Democratic leaders have said no vote is necessary.

The investigation is focused on the July call in which Mr Trump pressed Mr Zelensky to have Ukraine officials investigate Mr Biden and his businessman son, Hunter Biden, who had been on the board of a Ukrainian energy company.

Mr Hunter Biden, 49, denied doing anything improper in past work for the company but acknowledged he had benefited professionally from his father's political career in an interview with ABC News that aired on Tuesday.

Mr Joe Biden has denied Mr Trump's allegations of corruption.

Democrats have accused Mr Trump of pressuring a vulnerable US ally to dig up dirt on Mr Biden after withholding US$391 million (S$535 million) in US security aid intended to help combat Russian-backed separatists in the eastern part of Ukraine. Mr Zelensky agreed to investigate. Mr Trump eventually allowed the aid.

Mr Trump has denied wrongdoing and defended his request to Mr Zelensky during the phone call, which prompted a formal whistle-blower complaint by a person with a US intelligence agency.

If the Democratic-controlled House votes to approve articles of impeachment - formal charges - the Republican-controlled Senate would then hold a trial on whether to remove the President from office.

According to State Department, e-mails seen by Reuters, diplomat Mr Kent told colleagues that Ms Yovanovitch had become the target of a "classic disinformation operation".

Ms Yovanovitch last Friday denied Mr Giuliani's allegations that she provided a "do not prosecute list" to Ukrainian officials to protect Mr Biden and others.

"One key sign of it being fake is that most of the names are misspelled in English - we would never spell most that way," Mr Kent said in the e-mail to colleagues.