The US House of Representatives' impeachment probe into President Donald Trump intensified yesterday, as Trump raged about the inquiry and news reports suggested he had used additional diplomatic channels to go after his adversaries.

Three House committees said a subpoena for documents had been sent to Trump's lawyer Rudy Giuliani. The former New York mayor had said on television he asked the government of Ukraine to "target" former vice president Joe Biden, who is seeking the Democratic nomination to run against Trump in the 2020 election.