
US Secretary of State Mike Pompeo has been ordered by Democrats to turn over documents relating to the Trump administration's dealings with Ukraine.
In a letter, the heads of three House committees subpoenaed Mr Pompeo to produce the documents within a week.
It is the latest move in rapidly escalating impeachment proceedings against President Donald Trump.
He is being scrutinised for pushing Ukraine's president to investigate his Democratic rival Joe Biden.
In a separate development on Friday, the US special envoy for Ukraine negotiations, Kurt Volker, resigned, according to US media reports.
Washington has been rocked by a complaint from an intelligence whistleblower that Mr Trump was "using the power of his office to solicit interference from a foreign country in the 2020 US election".