Kurt Volker, the Unites States special representative on Ukraine who got caught in the middle of a whistle-blower complaint over President Donald Trump's dealings with Ukraine, has resigned, according to a US official.
Volker's departure is the first since the emergence of reports that Trump may have abused his presidential powers and sought help from a foreign government to undermine former Vice President Joe Biden, the current Democratic frontrunner, and help his own re-election.
The disclosures have prompted an impeachment inquiry, and Congress on Friday ordered Volker to answer questions in the investigation next week.
The official was quoted as saying by The Associated Press news agency that Volker told Secretary of State Mike Pompeo on Friday of his decision to leave the job, following disclosures that he had connected Trump's personal lawyer Rudy Giuliani with Ukrainian officials to investigate Biden and his family over alleged corrupt business dealings.
Giuliani has said he was in frequent contact with Volker about his efforts.
The State Department had no immediate comment on his resignation and has only said that Volker put Giuliani in touch with an aide to Ukraine's president.
Pompeo said on Thursday that as far as he knew, all State Department employees had acted appropriately in dealing with Ukraine.
Volker's resignation was first reported by the student newspaper at Arizona State University, where he directs an institute.
The State Press 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.
Committees in the Democratic-led House of Representatives ordered Volker to appear next Thursday to answer questions.
In a letter released on Friday, the legislators pointed to a tweet by Giuliani, in which he showed a screenshot of a conversation in which Volker spoke of connecting him with a top adviser to Ukraine's President Volodymyr 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," read the letter to Pompeo.
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.
As an envoy, Volker was tasked with overseeing critical US support to Ukraine as it faces a separatist conflict 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 $400m aid package for Ukraine as leverage to press for action on Biden.