More than two dozen Republican representatives surged into the room where US defence official Laura Cooper who oversees Ukraine and Russia matters, was due to testify.

The protesting Republicans yelled complaints that the Democrats running the inquiry were conducting the process in private.

Before the hearing room was stormed, dozens of Republicans gathered to denounce the impeachment process as a "joke," a "railroad job," a "charade" and "Soviet-style."

"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.

The dramatic confrontation caused a delay of around four hours, before Cooper began her testimony.

She was expected to face questions about Trump's decision this year to withhold $391 million in security assistance to Ukraine approved by Congress..

The impeachment probe is focusing on that decision alongside Donald Trump's request for Ukraine to investigate a domestic rival - former Vice President Joe Biden - for his personal political benefit.

"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."

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

Trump also criticised his Republican critics on Twitter, writing: "The Never Trumper Republicans, though on respirators with not many left, are in certain ways worse and more dangerous for our Country than the Do Nothing Democrats. Watch out for them, they are human scum!"

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

"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," Cicilline said.

The inquiry is being conducted in a secure room used to brief lawmakers about confidential or sensitive material.

Some of the Republicans brought mobile phones into the facility and refused to give them up, in violation of strict security rules

Australian Associated Press