A US impeachment inquiry descended into chaos on Thursday (Australian time) as a group of Republicans stormed into the hearing room and delayed testimony by a key Pentagon witness.

Urged by President Donald Trump to get tougher in fighting attempts to impeach him, more than two dozen Republican representatives surged into the room where US defence official Laura Cooper 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 delayed the testimony by Ms Cooper – who oversees Ukraine and Russia matters – by about four hours. A republican congressional aid said Capitol police were eventually called in to clear the room and bring order.

Ms Cooper was expected to face questions about Mr 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 Mr 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.”

Mr Trump on Monday said “Republicans have to get tougher and fight” the impeachment, saying the Democrats are “vicious and they stick together”.

He 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!”

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!

— Donald J. Trump (@realDonaldTrump) October 23, 2019

Democratic Representative David Cicilline saidholding the depositions in private protected 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,” Mr 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.

-with AAP