US Secretary of State Mike Pompeo has acknowledged that he was on the telephone call between Donald Trump and his Ukrainian counterpart that sparked impeachment proceedings against the Republican president.

Following a whistleblower complaint last week, Democrats are looking into Mr Trump's request to Ukrainian President Volodymyr Zelenskiy during a 25 July phone call to investigate former Vice President Joe Biden.

Mr Biden is a leading contender for the Democratic nomination to challenge Mr Trump in the 2020 presidential election.

"I was on the phone call," Mr Pompeo told reporters during a visit to Italy.

He said the call was in the context of US policy making in Ukraine, including "taking down the Russia threat," rooting out corruption in government and boosting the economy.

Mr Trump made the request to Mr Zelenskiy shortly after he had frozen nearly $400 million in US aid to Ukraine. Democrats accuse Mr Trump of soliciting foreign interference in a US election and of using taxpayer money as leverage for personal political gain.

The call and the whistleblower's complaint prompted House of Representatives Speaker Nancy Pelosi to launch a formal impeachment inquiry on 24 September.

In an intensifying battle between Congress and the executive branch, lawmakers have subpoenaed Mr Pompeo and Mr Trump's personal lawyer Rudolph Giuliani, as well as documents. They will begin depositions of current and former State Department officials this week.

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The department's inspector general, Steve Linick, was due to brief congressional staff today to address Ukraine-related documents that have been subpoenaed, according to two sources familiar with the situation.

Yesterday, Mr Pompeo objected to House efforts to obtain depositions and accused Democrats of bullying and intimidation.

Asked about his concerns, Mr Pompeo said State Department employees had been contacted directly by lawmakers or their staff and told not to talk to the State Department's legal counsel.

He said, however, that he would cooperate with Congress.

"We will of course do our constitutional duty to cooperate with this co-equal branch but we are going to do so in a way that is consistent with the fundamental values of the American system," he said.

The heads of three House committees leading the impeachment probe have, in turn, accused Mr Pompeo of intimidating witnesses, and said doing so was illegal.

The Democratic chairmen of the three committees warned Mr Pompeo yesterday that he would be considered "a fact witness in the House impeachment inquiry" if he was on Mr Trump's call with Mr Zelenskiy.

Mr Trump, who is seeking a second term in the election next year, has denied wrongdoing and assailed the impeachment probe.

According to a summary of the call released by the White House, Mr Trump asked Mr Zelenskiy during the July call to investigate Mr Biden and his son Hunter in coordination with US Attorney General William Barr and Giuliani.

Hunter Biden had sat on the board of a Ukrainian gas company that was under investigation by Kiev.

Kurt Volker, who resigned on Friday as Mr Trump's special representative for Ukraine, was to go to Capitol Hill to give a deposition to House staff on Thursday, the day he had been asked to appear.

Marie Yovanovitch, who was US ambassador to Ukraine until she was abruptly recalled in May, has agreed to appear on 11 October.

With their deep knowledge of Ukraine, testimony by Yovanovitch and Volker could be especially important to the impeachment probe.

The inquiry could lead to approval of articles of impeachment - or formal charges - against Mr Trump in the House. That would lead to a trial in the Senate on whether to remove him from office. But the president's fellow Republicans control that chamber and have shown little appetite for removing him.