Ambassador Bill Taylor testified before an impeachment inquiry that he was told Trump wanted Ukraine to probe ex-Vice-President Joe Biden"s dealings. Taylor said relations with Ukraine had been "fundamentally undermined".
President Trump insists there has been no such deal, or exchange of favours, in Ukraine policy.
Responding to the latest testimony in Congress, White House press secretary Stephanie Grisham said in a statement: "President Trump has done nothing wrong - this is a co-ordinated smear campaign from far-left lawmakers and radical unelected bureaucrats waging war on the Constitution".
Taylor, who has been the US envoy to Ukraine since June, is the latest witness to testify before lawmakers in the Democratic-led inquiry into suspected abuse of power by Donald Trump. A transcript of a call of President made to Volodymyr Zelensky on 25 July shows he urged him to investigate discredited corruption allegations against Biden and his son, Hunter.