Kurt Volker, special envoy for Ukraine, had helped plan a meeting between Giuliani and a Ukraine presidential aide The committees sent a separate note to Pompeo notifying him of a rapid schedule of depositions they expected to hold with five state department officials. Photograph: Drew Angerer/Getty ImagesKurt Volker, the US special envoy for Ukraine, has become the first casualty in the rapidly growing impeachment crisis surrounding Donald Trump, with multiple reports that he has resigned from his post.Volker was due to appear before the congressional committees and was mentioned in the whistleblower complaint as helping Ukraine president Volodymyr Zelenskiy “navigate” Trump’s demands.The news, first reported in an Arizona student newspaper, the State Press, emerged late on Friday, hours after Congress issued a subpoena to the US secretary of state, Mike Pompeo, to hand over documents related to contacts the president and his lawyer had with the Ukrainian government.It emerged this week that Volker had helped organise a meeting between Trump’s personal lawyer, Rudy Giuliani, and a Ukraine presidential aide. Giuliani was trying to convince Zelenskiy’s government to investigate of the son of Joe Biden, Trump’s possible opponent in next year’s elections.With impeachment proceedings against Trump under way, further reports emerged of White House efforts to limit access to transcripts of conversations with other countries. Trump’s phone calls with Saudi crown prince Mohammed bin Salman and Russian president Vladimir Putin were also tightly restricted, according to former administration officials quoted by CNN.The Washington Post reported that further restrictions were placed on details from a 2017 meeting between Trump, Russian foreign minister Sergei Lavrov and ambassador Sergei Kislyak, in which Trump is reported to have said he was not bothered by Moscow interference in the 2016 election because the US did the same kind of thing in other countries. The White House limited access to the remarks to “an unusually small number of people”, the Post said, after speaking to three former officials. The president had already been accused of sharing highly sensitive information on Islamic State during the meeting.Earlier on Friday, in a congressional letter delivered to Pompeo, three House committees demanded documents as part of their investigation into “the extent to which President Trump jeopardised national security by pressing Ukraine to interfere with our 2020 election and by withholding security assistance provided by Congress to help Ukraine counter Russian aggression”.The chairmen of the intelligence, foreign affairs and oversight committees also warned Pompeo that “your failure or refusal to comply with the subpoena shall constitute evidence of obstruction of the House’s impeachment inquiry”.The committees sent a separate note to Pompeo notifying him of a rapid schedule of depositions they expected to hold with five state department officials involved in contacts with Ukraine. The list begins next Wednesday with the former ambassador, Marie Yovanovitch, who was forced to retire from the post in May, earlier than planned.Trump, according to the White House version of his 25 July conversation with Zelenskiy, was scathing about Yovanovitch, referring to her as “the woman” who was “bad news”, and who was “going to go through some things”.Volker is due to appear before the day after Yovanovitch’s scheduled deposition. Giuliani has publicly displayed text messages purportedly from Volker, about a meeting had helped arrange with a Zelenskiy aide.Giuliani was seeking to persuade the Kyiv government to investigate Hunter Biden, the son of former vice-president Joe Biden, Trump’s possible Democratic opponent in next year’s presidential election.The younger Biden had been on the board of an Ukrainian energy company, but an Ukrainian investigation found no evidence of impropriety.Shortly before making the call to Zelenskiy in July, to press him further on investigating Biden, Trump ordered the suspension of US military aid to Ukraine.Speaking to reporters on Thursday, Pompeo claimed to have been too busy to read more than a couple paragraphs of the whistleblower complaint about Trump’s behaviour towards Ukraine, and insisted that no state department official had done anything appropriate.In their letter to Pompeo, the three committee chairman, noted that he had first been asked to hand over relevant documentation on 9 September, but had not complied.“Your actions are all the more troubling given that since our 9 September request, it has become clear that multiple state department officials have direct knowledge of the subject matters of the House’s impeachment inquiry,” the chairmen – Democrats Eliot Engel, Adam Schiff and Elijah Cummings – said.Pompeo’s continued refusal to hand over the documents, they added, would impair Congress’s “constitutional responsibilities to protect our national security and the integrity of our democracy”.