US Energy Secretary Rick Perry says the controversy over the Trump administration's handling of Ukraine has nothing to do with his resignation.
Perry also said he did not know whether he would co-operate with the Democrats' impeachment inquiry after his departure, which was announced by US President Donald Trump on Thursday.
He has found himself engulfed in Congress's impeachment investigation into whether the Republican president improperly withheld nearly $US400 million ($A584 million) in military aid for Ukraine in exchange for Kiev investigating his Democratic political rivals.
"It has absolutely nothing to do with Ukraine," Perry said of his planned departure in a Fox News interview, adding he was looking to return to Texas for "the next adventure in life".
US President Donald Trump listens as Energy Secretary Rick Perry speaks about the administration's environmental policies. Source: AFP
Last week, the three Democratic-led US House of Representatives committees leading the impeachment probe issued a subpoena for Perry to turn over documents by Friday regarding any role he played in Trump's alleged bid to pressure Ukraine.
Asked if he would meet House Democrats' deadline, Perry deferred to his legal team.
"Our general counsel is taking a look at that right now and when they send their answer at the end of the day, whatever their decision will be, I'll follow that," Perry said.
The impeachment probe centres on US President Donald Trump's alleged efforts to pressure Ukraine President Volodymyr Zelenskiy in a July 25 call to investigate former vice president Joe Biden and his son Hunter.
Joe Biden is a leading contender for the 2020 Democratic presidential nomination, and faces unsubstantiated allegations he improperly tried to aid his son Hunter's business interests in Ukraine.
Perry told Fox he had pushed Trump to make the July call, agreeing with a characterisation the president had made earlier.
Perry said he urged Trump to talk in the call about corruption and providing Ukraine an alternative to natural gas from Russia, in the form of shipments of US liquefied natural gas.
Biden never came up in any discussions Perry had with Zelenskiy or the White House, he said.
Perry also said he had no problems about being told to consult with Trump's personal lawyer Rudy Giuliani on Ukraine matters.
"I've worked with him. I trust him," Perry said.
