Kurt Volker, the US State Department's special envoy to Ukraine, resigned on Friday, following the release of the declassified version of a whistleblower complaint at the center of Democratic lawmakers' inquiries on impeaching President Donald Trump.

Volker, was mentioned in the complaint that was released Thursday morning.

According to a section titled "ongoing concerns," Volker met with Ukrainian leaders to help "navigate" Trump's demands for Ukrainian President Volodymyr Zelensky after a July 25 phone call.

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Kurt Volker, the US State Department's special envoy to Ukraine, resigned on Friday, following the release of the declassified version of a whistleblower complaint at the center of Democratic lawmakers' inquiries on impeaching President Donald Trump.

Volker, was mentioned in the complaint that was released Thursday morning. According to a section titled "ongoing concerns," Volker met with Ukrainian leaders to help "navigate" Trump's demands for Ukrainian President Volodymyr Zelensky after a July 25 phone call. Volker was said to have been accompanied by Gordon Sondland, the ambassador to the European Union.

According to the whistleblower, who spoke with numerous US officials who looked at readouts of the meetings, the two US diplomats allegedly "provided advice" to the Ukrainian leaders on Trump's "demands."

Volker served part-time in his position, according to The New York Times, and served as the US ambassador to NATO. His resignation was first reported by The State Press, Arizona State University's student-run newspaper.

House lawmakers issued a deposition for Volker, in addition to other US officials mentioned in the whistleblower complaint, to testify before Congress next week.

Read more: Trump and his allies believe the Ukraine phone call was a nothingburger, and led some people to describe it as 'one of his better' calls with a foreign leader

Volker's resignation comes after a whistleblower brought to light the existence of a controversial phone call between the US and Ukrainian president. In a publicly released summary of the Trump-Zelensky phone call, the US president was said to have asked the Ukrainian president for a "favor" in investigating a conspiracy theory about the Democratic National Committee during the 2016 US presidential election.

Trump had also requested Zelensky to "look into" unproven allegations of misconduct from former Vice President and 2020 Democratic candidate Joe Biden and his son Hunter, according to the summary.

Acting Director of National Intelligence Joseph Maguire testified on Capitol Hill on Thursday, and the anonymous whistleblower has also indicated that they want to testify about the potential misconduct, according to information obtained by CNN.

Trump denied there was a "quid pro quo" arrangement during the call with President Zelensky, who met the US president in New York on Wednesday. Zelensky publicly echoed Trump's description of a cordial call, and he said he did not want to be involved in the "democratic, open elections of USA."