A US envoy has said that Secretary of State Mike Pompeo directed him to coordinate Ukraine policy with President Donald Trump's lawyer, further implicating the top US diplomat in the impeachment drama.

Gordon Sondland - the US ambassador to the European Union who, unlike previous diplomats who testified in the inquiry, is a political appointee allied with Mr Trump - said he kept Mr Pompeo up-to-date on what he said was the president's effort to set conditions to a US meeting with Ukraine's new leader.

Asked if Mr Pompeo had been made aware that Mr Trump wanted a Ukrainian investigation of domestic rival Joe Biden before agreeing to the White House meeting and releasing security aid, Mr Sondland replied: "Yes."

Representative Adam Schiff probed further, asking if Mr Pompeo denied a connection. "Not that I can recall," Mr Sondland replied.

Adam Schiff presides at the House Intelligence Committee hearing

Mr Sondland said Mr Pompeo had directed the US pointman on Ukraine, Kurt Volker, to speak to Donald Trump's personal lawyer Rudy Giuliani, the former New York mayor who was pushing Ukraine to investigate Joe Biden.

He said Mr Pompeo's stance did not change even after diplomats complained that Mr Giuliani was meeting with an allegedly corrupt Ukrainian prosecutor without their knowledge.

"Even as late as 24 September, Secretary Pompeo was directing Kurt Volker to speak with Rudy Giuliani," Mr Sondland said.

That is the same day that the White House released a July call in which Mr Trump pressed Ukrainian President Volodymyr Zelensky to "do us a favour," leading Democrats to move to impeach him.

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Mr Pompeo has repeatedly denounced the impeachment inquiry and has drawn fire from former diplomats for not defending career employees who have been caught up in the scandal and at times personally attacked by Donald Trump.

The Secretary of State, coincidentally in Brussels as Mr Sondland appeared in Washington, ignored reporters' questions on the ambassador's testimony.

Mr Sondland, a hotel owner who was appointed after donating to the Trump campaign, said the State Department and White House refused to share documents as he complied with an order to appear before Congress.

"These documents are not classified and, in fairness, should have been made available," he said.

"I have no doubt that a more fair, open, and orderly process of allowing me to read the State Department records and other materials would have made this process more transparent," he added.

Gordon Sondland testifying before the committee

The committee's top Republican, Devin Nunes, accused Democrats of waging an "impeachment crusade" against President Trump.

"They know exactly what kind of damage they're inflicting on this nation. But they've passed the point of no return," Mr Nunes said.

Adam Schiff, the Intelligence Committee's Democratic chairman, called Mr Sondland's testimony "a very important moment in the history of this inquiry."