Washington: Kurt Volker, the State Department's special envoy for Ukraine who got caught in the middle of the pressure campaign by President Donald Trump and his lawyer, Rudy Giuliani, to find damaging information about Democrats, abruptly resigned his post Friday.

Volker, who told Secretary of State Mike Pompeo on Friday that he was stepping down, offered no public explanation, but a person informed about his decision said he concluded it was impossible to be effective in his assignment given the developments of recent days.

His departure was the first resignation since revelations about Trump's efforts to pressure Ukraine's president to investigate former vice president Joe Biden and other Democrats. The disclosures have triggered a full-blown House impeachment inquiry, and House leaders announced Friday that they planned to interview Volker in a deposition next week.

Volker was former ambassador to NATO who served in the part-time, unpaid position to help Ukraine resolve its armed confrontation with Russian-sponsored separatists.