Mike Pompeo did not say what was discussed in Donald Trump's conversation with Volodymyr Zelensky.

Secretary of State Mike Pompeo acknowledged on Wednesday that he listened in on the telephone call between Donald Trump and his Ukrainian counterpart that prompted a House of Representatives impeachment inquiry against the Republican U.S. president.

The admission by Pompeo, a key Trump ally, moved the top U.S. diplomat closer to the center of the scandal and raised further questions about his role in the administration's interactions with Ukraine, including the recall of the U.S. ambassador to Kiev earlier this year.

The Democratic-led House last week launched its impeachment inquiry, which threatens Trump's presidency, in the aftermath of a complaint brought by a whistleblower within the U.S. intelligence community over Trump's request to Ukrainian President Volodymyr Zelenskiy during a July 25 phone call to investigate former Vice President Joe Biden.

Biden is a leading contender for the Democratic nomination to challenge Trump in the 2020 presidential election. Democrats have accused Trump of pressuring a vulnerable U.S. ally to interfere in the 2020 U.S. election to harm a domestic political rival for Trump's personal political benefit.

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

Pompeo said the call was in the context of U.S. policymaking in Ukraine, including "taking down the Russia threat," rooting out corruption in government and boosting the economy. The Wall Street Journal first reported that Pompeo was on the call.

Trump made his request to Zelenskiy shortly after he had frozen nearly $400 million in U.S. aid to Ukraine. Zelenskiy agreed to Trump's request on the call. The aid money was later provided to Ukraine. Democrats have accused Trump of using taxpayer money as leverage on Ukraine.

Trump, who is seeking a second four-year term as president, has denied wrongdoing and has assailed the impeachment probe.

In an intensifying battle between House Democrats and Trump's administration, Democratic-led committees have subpoenaed Pompeo and Trump's personal lawyer Rudy Giuliani, as well as documents. They will begin depositions of current and former State Department officials this week.

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

'Fundamental Values'

Pompeo on Tuesday objected to House efforts to obtain depositions and accused Democrats of bullying and intimidation.

Asked about his concerns, 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," Pompeo said.

The Democratic chairmen of three House committees have accused Pompeo of intimidating witnesses, and said doing so is illegal. They warned Pompeo on Tuesday that he is considered "a fact witness in the House impeachment inquiry" based on his role in Trump's call with Zelenskiy.

House Oversight Committee Chairman Elijah Cummings intends to subpoena the White House for Ukraine-related records on Friday after what he described as "flagrant disregard of multiple voluntary requests for documents," according to a memo he sent to the panel.

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

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

The prospect that Trump solicited Ukraine's help against his potential challenger next year has infuriated Democrats, many of whom blame the loss of the 2016 presidential election on Russian interference. Moscow has denied interfering in that campaign.

On Wednesday, Russian President Vladimir Putin said he saw no evidence of pressure in Trump's July conversation with Zelenskiy and added that there was nothing wrong with the U.S. president asking for an investigation into potential corruption.

U.S. intelligence agencies and Special Counsel Robert Mueller concluded that Russia interfered in the 2016 election with a scheme of hacking and propaganda to boost the Republican Trump's candidacy and disparage his Democratic opponent Hillary Clinton.

Kurt Volker, who resigned last Friday as Trump's special representative for Ukraine, was scheduled 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 U.S. ambassador to Ukraine until she was abruptly recalled in May, has agreed to appear on Oct. 11. In his phone call with Zelenskiy, Trump called Yovanovitch "bad news."

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 Trump in the House. That would lead to a trial in the Senate on whether to remove him from office. The president's fellow Republicans control the Senate and have shown little appetite for removing him.