US special representative on Ukraine Kurt Volker resigned on 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 favourite 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 on 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.Volker is a veteran diplomat involved in Europe who was appointed US ambassador to NATO under former president George W Bush. He left the diplomatic service to become a consultant and in 2012 was named executive director of Arizona State University's McCain Institute, a centre focused on national security named after senator John McCain.The Trump administration in 2017 appointed Volker to take charge of US policy on Ukraine, in an unusual arrangement in which he was essentially a volunteer for the State Department while maintaining his university duties. The State Press, the student-run publication at Arizona State University, quoted a university spokesperson as saying that Volker had quit his Ukraine position. The Arizona Republic also quoted the university's president, Michael Crow, as confirming that Volker would stay at the institute but leave the State Department. The State Department did not answer requests for comment.As an envoy, Volker was tasked with overseeing critical US support to Ukraine as it faces a separatist insurgency backed by Russia.More than 13 000 people have died since fighting broke out in 2014, when Russia also annexed the Crimean peninsula from Ukraine.Democrats are looking into whether Trump used a delay in a $400 million aid package for Ukraine as leverage to press for action on Biden.