Trump's Ukraine envoy Volker resigned.

Washington (AFP) - 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 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.