WASHINGTON (NYTIMES) - A National Security Council aide testified on Thursday (Oct 31) that a top diplomat who was close to President Donald Trump told him that a package of military assistance for Ukraine would not be released until the country committed to investigating Trump's political rivals, corroborating a key episode at the centre of the impeachment inquiry.

The closed-door deposition by Timothy Morrison, who announced his resignation on Wednesday on the eve of his appearance before impeachment investigators, suggests that a Trump-appointed ambassador proposed a quid pro quo in which security assistance money allocated by Congress would be provided only in exchange for the political investigations the president was seeking.

His account confirmed the one given last week by Ambassador William Taylor, the top US diplomat in Ukraine, during his private testimony.

Morrison briefed Taylor on a series of communications involving the president and his ambassador to the European Union, Gordon Sondland, according to his prepared remarks for Thursday's appearance, which was reviewed by The New York Times.

Taylor testified last week that Morrison, a top Russia and Europe expert, had informed him in early September of a meeting in Warsaw, Poland, between Sondland and a top aide to President Volodymyr Zelenskiy of Ukraine.

Sondland told the Ukrainian aide the United States would only provide a package of US$391 million (S$530 million) in security assistance that Congress had allocated for the country if Zelenskiy committed to investigate allegations related to former vice-president Joe Biden and his son, who sat on the board of a Ukrainian energy company.

A publicly available transcript already shows Trump pressed Zelenskiy to undertake the investigations of Democrats.

But investigators are trying to establish whether Trump used the security aid and a coveted White House meeting with Zelenskiy as leverage in a pressure campaign to secure the inquiries.

Morrison's testimony came as Democrats were moving to wrap up their closed portion of their inquiry in the coming week or so.

As he met with investigators, they muscled through a resolution on the floor of the House endorsing the inquiry and laying out a path to move their work into the open and begin a debate over impeachment articles in the coming weeks.

Republicans uniformly opposed the measure, which they said fell short of redeeming an illegitimate, politically motivated crusade by Democrats to undo the 2016 election.