US President Donald Trump's personal lawyer Rudy Giuliani today said he would not comply with a subpoena issued by US House of Representatives Democrats as part of their impeachment inquiry into the president.

A letter from Mr Giuliani's lawyer, Jon Sale, told a lawyer for the three Democratic-led House committees leading the inquiry that the former New York mayor would refuse to comply with the subpoena seeking documents relating to his activities concerning Ukraine.

"This appears to be an unconstitutional, baseless, and illegitimate 'impeachment inquiry,'" Mr Sale wrote.

The letter echoes language the White House used last week in saying it would not co-operate with the inquiry that threatens Mr Trump's presidency.

The move represented the latest effort by Mr Trump and those close to him to refuse any co-operation with the inquiry.

US Vice President Mike Pence has also declined to submit documents to the committees pursuing the impeachment inquiry, his office has said.

Mr Pence's counsel, Matthew Morgan, said in a letter to the politicians leading the inquiry that the scope of documents requested included some "which are clearly not vice-presidential records, pursuant to a self-proclaimed 'impeachment inquiry.'"

Mr Morgan said Mr Pence's office was prepared to work with the committees if they "wish to return to the regular order of legitimate legislative oversight requests, and the Committees have appropriate requests for information solely in the custody of the Office of the Vice President."

Despite that, several US officials involved in the matter who received subpoenas from the three committees have testified.

Even as Mr Giuliani refused to comply, politicians were hearing closed-door testimony from a senior US diplomat, George Kent.

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House Democrats are focusing on Mr Trump's request to Ukrainian President Volodymyr Zelenskiy in a phone call on 25 July to look into unsubstantiated allegations about Joe Biden, the former vice president and a leading contender to become the Democratic nominee to run against Republican Trump in next year's presidential election.

Mr Kent, who has spent much of his career fighting corruption in Ukraine and elsewhere, is the second career diplomat to testify as part of the probe after being subpoenaed.

The White House and State Department had ordered them not to appear.

It was unclear what Mr Kent, a deputy assistant secretary of state responsible for US policy toward six former Soviet republics including Ukraine, was telling politicians.

According to the New York Times, he raised concerns with colleagues as far back as March about Mr Giuliani pressuring Ukraine to pursue investigations into Mr Trump's political rivals.

Marie Yovanovitch, the former US ambassador to Ukraine, on Friday accused the Trump administration in testimony of recalling her in May based on false claims.

Fiona Hill, Mr Trump's former Russia adviser, told politicians yesterday that she and her then-boss, former national security adviser John Bolton, were alarmed this summer by efforts to force Ukraine to investigate Mr Biden and other rivals and advised her to notify a National Security Council lawyer, according to a source familiar with her testimony.