William Taylor, top US diplomat in Ukraine and key impeachment witness, is stepping down
In a brief e-mail to The New York Times, Mr Taylor said he would leave in early January because his temporary appointment to Ukraine last June is set to expire.
Under the Vacancies Act, political appointees in an acting position can hold office only for about 200 days. Earlier in the day, people familiar with the planning had suggested he would leave by the end of December.
"The administration will nominate a permanent ambassador soon," Mr Taylor said. He did not elaborate.
Mr Taylor served as something of a star witness for the House Intelligence Committee's impeachment inquiry against Mr Trump.
In public testimony last month, he calmly and confidently recounted for lawmakers what he described as a pressure campaign by the Trump administration to leverage American security aid to Ukraine in exchange for an investigation into Mr Trump's political opponents.
"Security was so important for Ukraine, as well as our own national interests," Mr Taylor testified at the Nov 13 hearing.
"To withhold that assistance for no good reason other than help with a political campaign made no sense. It was counterproductive to all of what we had been trying to do. It was illogical. It could not be explained. It was crazy."
At the hearing, Mr Taylor described a growing sense of alarm at learning that US$391 million (S$530 million) in military aid for Ukraine had been held up.
He also said he had discovered that Mr Trump was conditioning "everything" about the United States' relationship with Ukraine - including a White House meeting for Ukraine's president - on the country's willingness to commit publicly to investigations of his political rivals.
In a July 25 telephone call, Mr Trump pressed President Volodymyr Zelensky of Ukraine to investigate two politically charged allegations: one was a widely debunked conspiracy theory about Ukrainian involvement in 2016 election tampering, and the other was related to corruption at an energy company that employed the younger son of former vice-president Joe Biden, Mr Hunter Biden.
There is no evidence that the Bidens were involved in wrongdoing.
Mr Taylor, a long-time diplomat, was asked to come out of retirement after the US ambassador to Kiev, Ms Marie Yovanovitch, was ousted for resisting a shadow foreign policy campaign in Ukraine that was run by Mr Trump's personal lawyer Rudy Giuliani.
But after Mr Taylor was pulled into what he called an "irregular channel" of diplomatic relations between Washington and Kiev, he became one of the most senior State Department officials to openly challenge it.
Withholding the security aid as threatened would be "crazy", Mr Taylor wrote on Sept 9 in a text to Mr Gordon Sondland, the US ambassador to the European Union.
Mr Taylor also threatened to quit if Ukrainian officials committed to an investigation of Mr Trump's rivals and still did not receive the US$391 million in aid - what Mr Taylor called a "nightmare" situation.
He was recalled to the State Department from a position where he helped lead the US Institute of Peace after nearly five decades of government work - including a tour in Vietnam as an Army infantry soldier, a stint as a Senate staff member and diplomatic postings including Brussels, Baghdad and Kabul, Afghanistan.
But he was expected to serve in Kiev only temporarily.
Names already are being rumoured for Mr Taylor's replacement, including retired lieutenant general Keith Dayton, according to a person familiar with the issue.
Mr Dayton is the director of the George C. Marshall European Centre for Security Studies, in Germany, but was appointed in 2018 as the senior US defence adviser to Ukraine, according to his biography.