US special envoy to Ukraine Kurt Volker has resigned amid controversy over a whistleblower complaint concerning US president Donald Trump's dealings with the country.
The move came as House Democrats took their first concrete steps in the impeachment investigation of Mr Trump, issuing subpoenas demanding documents from Mike Pompeo and scheduling legal depositions for other State Department officials.
A US official, who spoke on condition of anonymity, said Mr Volker told secretary of state Mike Pompeo on Friday of his decision to leave the job.
The move followed disclosures Mr Volker had connected Mr Trump's personal lawyer Rudy Giuliani with Ukrainian officials to investigate former vice president Joe Biden and his family over allegedly corrupt business dealings.
The State Department had no immediate comment on Mr Volker's resignation and has said only that he put Mr Giuliani in touch with an aide to Ukraine's president Volodymyr Zelenskiy.
News of the House Democrats issuing subpoenas came at the end of a stormy week of revelation and recrimination, with House Speaker Nancy Pelosi framing the impeachment inquiry as a sombre moment for a divided nation.
Mr Trump, for his part, insisted anew that his actions and words had been “perfect” and the whistleblower’s complaint about his comments to the leader of Ukraine might well be the work of “a partisan operative”.
Meanwhile, Mr Trump has been warned against retaliating as he lashed out at anyone who might have helped the intelligence whistleblower whose complaint has led to the impeachment probe launched against him this week.
The whistleblower’s complaint alleged the president abused the power of his office to “solicit interference from a foreign country” in next year’s US election.
In a July 25th phone call, days after ordering a freeze to some military assistance for Ukraine, Mr Trump prodded the new president Zelenskiy to investigate Democratic rival Mr Biden, and volunteered the assistance of Mr Giuliani and attorney general William Barr.
Late on Thursday, Mr Trump denounced people who might have talked to the whistleblower as “close to a spy” and suggested they engaged in treason, an act punishable by death.
On Friday, he targeted the complainant, a CIA officer, tweeting: “Sounding more and more like the so-called Whistleblower isn’t a Whistleblower at all.”
Ms Pelosi, speaker of the House of Representatives, told MSNBC’s Morning Joe show: “I’m concerned about some of the president’s comments about the whistleblower.”
She said House panels conducting the impeachment probe will make sure there is no retaliation against people who provided information in the case.
On Thursday, House Democratic chairmen called Mr Trump’s comments “witness intimidation” and suggested efforts by him to interfere with the potential witness could be unlawful.
Mr Trump's Friday comment questioning the whistleblower's status could foreshadow an effort to argue that legal protection laws do not apply to the person, opening a new front in the president's battles with Congress.
The intelligence community’s inspector general found the whistleblower’s complaint “credible” despite finding indications of the person’s support for a different political candidate.