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 allegedly pressuring Ukraine’s president to investigate Democratic rival Joe Biden.
In a separate development on Friday, the US special envoy for Ukraine negotiations, Kurt Volker, resigned, US media reported.
Mr Trump has denied putting any pressure on Ukrainian President Volodymyr Zelensky in a phone call in July, when Mr Biden was leading polls to win the Democratic nomination for the White House race in 2020.
Mr Trump has alleged that Mr Biden pressed for the sacking of Ukrainian prosecutor Viktor Shokin in 2016 to protect a business that employed his son, Hunter Biden.
Mr Biden did call for the sacking of Mr Shokin, even threatening to withhold $1bn (£813m) in aid to Ukraine. But so did a number of other Western officials who saw Mr Shokin as a hindrance to anti-corruption investigations.
Impeachment is a two-stage political process, rarely exercised, by which a US president can be removed from office for wrongdoing.
Even if President Trump were impeached by the House of Representatives, he is unlikely to be forced out of the White House because Republicans control the Senate.
Source: BBC
