Gordon Sondland, the US ambassador to the European Union and a key figure in the Democratic-led impeachment inquiry against Trump, depicted Pompeo as a participant in the president's efforts to get Ukraine to carry out investigations that could help him politically.

While Trump is not expected to oust one of his staunchest supporters over the testimony, Pompeo has been widely reported to be mulling his exit to run for the US Senate from his home state of Kansas next year.

Two senior US officials said the testimony puts Pompeo in a more difficult position with both his department's career staff and international counterparts that could hasten his departure.

He has already faced internal criticism for his refusal to cooperate with the impeachment inquiry and failure to defend career Foreign Service officers who have testified and have been targeted by smears.

"It's hard to see him riding this out," said one of the officials, speaking on condition of anonymity.

Asked how much Sondland's testimony could hurt Pompeo or accelerate his departure, the other official, a senior US diplomat, said on condition of anonymity, "Everyone is asking the same question."

"He's in an unsustainable position," added a former senior US diplomat, who requested anonymity.

The State Department declined to comment when asked about Pompeo's future.

Speculation has swirled for months that Pompeo would resign, fuelled by his frequent visits to Kansas and regional media interviews. Some administration officials had predicted privately he would wait until shortly before the Kansas filing deadline at the start of June.

Pompeo has been a close Trump ally, serving as CIA director before becoming the top US diplomat. Despite their vaunted personal chemistry, strains have developed between them recently as Trump has complained in private about Pompeo's failure to prevent top diplomats from testifying in the impeachment probe hearings, the senior administration official said.

"Everyone was in the loop. It was no secret," Sondland testified. "We kept the State Department leadership and others apprised of what we were doing."

Witnesses in the impeachment inquiry, including Sondland, have said they believe Trump was pressuring Ukraine to commit to investigating Democratic rival Joe Biden and his son, Hunter, by withholding an invitation for an Oval Office visit, and by temporarily freezing nearly $US400 million in US security aid that Ukraine needs to defend against Russia-backed separatists.

Asked by a reporter at a news conference on Wednesday at NATO headquarters in Brussels to comment on Sondland's testimony, Pompeo replied testily, "I did not see a single thing, I was working. Sounds like you might not have been."

Sondland denied participating in "rogue diplomacy." He read from emails that he said "show that the leadership of State, NSC and the White House were all informed about the Ukraine efforts" from late May until the US security aid was released to Ukraine in September.

"NSC" stands for the White House's National Security Council.

Sondland cited a July 19 email he sent to Pompeo and others informing them Ukraine's President Volodymyr Zelenskiy was ready to tell Trump in a July 25 call that he "intends to run a fully transparent investigation and will 'turn over every stone.'"

Australian Associated Press