WASHINGTON (REUTERS) - Republican lawmakers, encouraged by President Donald Trump to get tougher in fighting Democratic efforts to impeach him, disrupted the United States House of Representatives impeachment inquiry on Wednesday (Oct 23) by storming into a high-security hearing room and delaying testimony by a key Pentagon witness.

More than two dozen Republican lawmakers surged into the facility where Ms Laura Cooper, the US defence official who oversees Ukraine and Russia matters, was due to testify behind closed doors and began yelling complaints that the Democrats were conducting the process in private, lawmakers and aides said.

After a delay of around four hours, Ms Cooper began her testimony.

In a dramatic confrontation during the escalating probe that threatens Mr Trump's presidency even as he seeks re-election next year, the Republican lawmakers caused a stand-off with the three Democratic-led House committees leading the impeachment inquiry before finally leaving the room to take part in a vote on unrelated legislation on the House floor.

The impeachment inquiry focuses on Mr Trump's request for Ukraine to investigate a domestic rival - Democrat Joe Biden - for his personal political benefit.

By having Republican lawmakers who are not members of the relevant committees cause disarray by barging into the hearing room, Mr Trump's allies sought to put the focus on what they portray as unfair Democratic tactics rather than on the President's conduct.

"The American people have a voice in this process. They have a right to know. It should be in the sunlight," House Republican Leader Kevin McCarthy told reporters.

Although the Republicans complain of a lack of transparency in the inquiry, the US Constitution gives the House wide latitude in how to conduct the impeachment process and set rules for the probe.

"They're freaked out. They're trying to stop this investigation," Democratic Representative Ted Lieu said of the Republicans. "They know more facts are going to be delivered which are absolutely damning to the President of the United States."

A witness inside the room said the Republicans brought mobile phones into the facility even though electronic devices are forbidden. An Intelligence Committee official said some Republicans refused to remove their phones. The House parliamentarian ruled that the Republican lawmakers violated House rules, the official added.

Intelligence Committee chairman Adam Schiff told reporters that the witnesses testifying in the inquiry have defied the White House efforts to keep them silent and that "the President has urged his acolytes in Congress to use other means to try to prevent their testimony. But they won't be successful".

Mr Trump on Monday told reporters that "Republicans have to get tougher and fight" the impeachment, saying the Democrats are "vicious and they stick together".

"It never ends. The Do Nothing Dems are terrible!" he wrote on Twitter earlier on Wednesday, later adding their "case is DEAD!"

Before the hearing room was stormed, dozens of House Republicans appeared before reporters, with some denouncing the impeachment process run by Democrats as a "joke," a "railroad job", a "charade" and "Soviet-style".

They complained that testimony was being taken privately rather than in public hearings and that the House did not hold a vote formally authorising the investigation.

Republicans who are members of the three committees have taken part in the process throughout.

WITNESS TESTIFIES

Ms Cooper, the deputy assistant secretary of defence for Russia, Ukraine and Eurasia, began testifying on Wednesday afternoon after the stand-off ended, an Intelligence Committee official said. She was expected to face questions about Mr Trump's decision this year to withhold US$391 million (S$533 million) in security assistance to Ukraine approved by Congress.

In testimony to the inquiry on Tuesday, Mr William Taylor, the top US diplomat in Ukraine, said Mr Trump had made the aid contingent on Ukraine announcing it would conduct politically motivated investigations the President demanded.

So far, few Republicans have appeared inclined toward Mr Trump's removal, though there have been some cracks in their support. Senator John Thune, the Senate's No. 2 Republican, told reporters that the picture painted by Mr Taylor's testimony "based on the reporting that we've seen is not a good one".

The inquiry could lead to the House passing formal charges known as articles of impeachment, prompting a trial in the Republican-controlled Senate on whether to remove Mr Trump from office.

Democratic Representative David Cicilline told reporters that holding the depositions in private protects the integrity of the inquiry.

"The investigation is akin to a grand jury where you collect evidence. One of the reasons you do these in private is because you want to prevent witnesses from attempting to align their testimony to the testimony of another witness by watching it or reading a transcript. It's how you protect the integrity of any investigation," Mr Cicilline said.

Democratic Representative Eric Swalwell added: "We see this (Wednesday's disruption) as an effort not only to intimidate this witness but also to intimidate future witnesses from coming forward. It's not going to work. We're not going to be deterred."

Mr Taylor testified that he was told by the US envoy to the European Union, that Mr Trump had linked the aid's release to public declarations by Ukrainian President Volodymyr Zelensky that he would investigate Mr Biden, his son Hunter Biden's tenure on the board of a Ukrainian energy company called Burisma, and a debunked conspiracy theory that Ukraine, and not Russia, meddled in the 2016 US presidential election.

The impeachment inquiry, triggered by a whistle-blower complaint against Mr Trump by a person within the US intelligence community, focuses on a July 25 telephone call in which Mr Trump asked Mr Zelensky to carry out those two investigations. Mr Zelensky agreed during the call. The aid was later provided.

Federal election law prohibits candidates from accepting foreign help in an election.

The Democratic heads of the committees leading the inquiry said the State Department refused to comply with a subpoena for documents, which included memoranda on efforts to intimidate employees, as well as communications on Mr Trump's request to Ukraine to conduct political investigations.

The department's action "indicates that these documents support the allegations against the President and others", the lawmakers wrote in a letter to a department official.