US special representative on Ukraine Kurt Volker resigned Friday after Congress ordered him to answer questions in an impeachment investigation on President Donald Trump, a source said.

A person familiar with the matter who requested anonymity confirmed Volker's resignation, which was first reported by the student newspaper at Arizona State University, where he directs an institute.

A whistleblower complaint released on Thursday said that Volker met senior Ukrainian officials on how to "navigate" Trump's demands of President Volodymyr Zelensky.

The complaint accused Trump of pressuring Zelensky in a July 25 phone call to supply dirt on former US vice president Joe Biden, the favorite to represent Democrats against Trump in the 2020 presidential election.

Committees in the Democratic-led House of Representatives ordered Volker to appear next Thursday to answer questions.

In a letter released Friday, the lawmakers pointed to a tweet by Rudy Giuliani, Trump's personal lawyer, in which he showed a screenshot of a conversation in which Volker spoke of connecting him with a top adviser to Zelensky.

"The failure of any of these Department employees to appear for their scheduled depositions shall constitute evidence of obstruction of the House's impeachment inquiry," said the letter to Secretary of State Mike Pompeo.

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