New York: In explosive testimony to the impeachment inquiry into US President Donald Trump, the US ambassador to the European Union confirmed there was an explicit "quid pro quo" linking a White House visit to politically motivated investigations.

Gordon Sondland, a Trump appointee and donor, said the highest levels of the White House and State Department were involved in the push for Ukraine to announce investigations into the family of potential 2020 rival Joe Biden and into debunked conspiracy theories about the 2016 election.

A senior US diplomat told lawmakers that President Donald Trump expressly ordered him and others to help pressure Ukraine into investigating a political rival of the president, providing some of the most significant testimony to date in the House of

This level of involvement included Secretary of State Mike Pompeo, Vice-President Mike Pence and former national security adviser John Bolton.

"They knew what we were doing and why," Sondland told the House Intelligence Committee on Wednesday, local time (Thursday AEDT). "Everyone was in the loop. It was no secret."

He stressed that his efforts to pressure Ukraine were at "the express direction" of the President, who authorised his personal lawyer Rudy Giuliani to act on his behalf.

"The suggestion that we were engaged in some irregular or rogue diplomacy is absolutely false," he said, citing a trove of WhatsApp messages with senior officials he brought with him to the hearings.

"We all understood that if we refused to work with Mr Giuliani, we would lose an important opportunity to cement relations between the United States and Ukraine.

"So I followed the President's orders."

Sondland's testimony was highly anticipated because, unlike other witnesses who have appeared so far, he was in regular contact with the President during the period in question.

Earlier this month he amended his previous testimony at a private deposition in which he claimed there was no quid pro quo.

"I know that members of this Committee have frequently framed these complicated issues in the form of a simple question: Was there a 'quid pro quo?'" Sondland said in his opening statement.

"As I testified previously, with regard to the requested White House call and White House meeting, the answer is yes."

Sondland said he also came to believe that almost $US400 million ($587 million) in vital security aid would not be released to Ukraine until the investigations were announced.

He testified that Trump did not tell him this directly, but that he became "absolutely convinced" there was a link between the investigations and the release of the aid.

“President Trump never told me directly that the aid was conditioned on the investigations," Sondland said under questioning.

"The aid was my own personal guess based, again, on your analogy, two plus two equals four.”

Sondland said it was crucial that Ukraine announce the investigations publicly, but they did not necessarily have to conduct them.

Trump said he did not know Sondland well, but he "seems like a nice guy, though". AP

Speaking on the White House lawn during Sondland's testimony, Trump told reporters: "I have not spoken to him much.

"This is not a man I know well.

"Seems like a nice guy, though."

Trump read aloud from a section of Sondland’s deposition in which he described a September phone call when the President told him he did not want a quid pro quo.

"That means it’s all over," Trump said.

"This is the final word from the President of the US.

"I want nothing.”

White House press secretary Stephanie Grisham said that "no quid pro quo ever occurred".

"Ambassador Sondland’s testimony made clear that in one of the few brief phone calls he had with President Trump, the president clearly stated that he ‘wanted nothing’ from Ukraine and repeated ‘no quid pro quo over and over again,’" Grisham said.

In the afternoon, State Department official Laura Cooper said Ukrainian officials had raised concerns with her staff about the withheld military aid on July 25 - the same day of the phone call in question and earlier than had previously thought.

Adam Schiff, the Democratic chair of the House Intelligence Committee, said: "I think today's testimony is among the most significant evidence to date.

"It goes right to the heart of the issue of bribery as well as other potential high crimes or misdemeanours."