WASHINGTON -- A former top White House official confirmed that military aid to Ukraine was held up by U.S. President Donald Trump's demand for the ally to investigate Democrats and Joe Biden but testified that there's nothing illegal, in his view, about the quid pro quo at the centre of the Democrat-led impeachment inquiry.

Tim Morrison, who stepped down from the National Security Council the day before his Thursday testimony, was the first White House political appointee to appear and spent more than eight hours behind closed doors with House investigators.

"I want to be clear, I was not concerned that anything illegal was discussed," Morrison said about a pivotal phone call between Trump and the Ukraine president, according to prepared remarks obtained by The Associated Press.

Late Thursday, Trump tweeted about Morrison's comment that no law was broken: "Thank you to Tim Morrison for your honesty."

But Morrison also confirmed what diplomat William Taylor told investigators in earlier testimony -- that Morrison had a "sinking feeling" when he learned that Trump was asking the Ukrainians to publicly announce an investigation of Biden and the Democrats, even as the Republican president denied it was a quid pro quo.

"I can confirm," Morrison wrote, that the substance of the diplomat's testimony "is accurate."

Morrison told investigators that he and Taylor did not realize the money was being withheld for the investigation of Burisma, the gas company connected to Biden, until a conversation with European Union Ambassador Gordon Sondland in September.

"Taylor and I had no reason to believe that the release of the security sector assistance might be conditioned on a public statement reopening the Burisma investigation until my Sept. 1, 2019, conversation with Ambassador Sondland," Morrison testified.

A defence hawk, Morrison was the National Security Council's top adviser for Russian and European affairs until he stepped down Wednesday. He was brought into the White House by John Bolton, the former national security adviser who was critical of Trump's Ukraine policy and the back-channel diplomacy being run by the Republican president's personal lawyer Rudy Giuliani.

Morrison testified that he was told by his predecessor, Fiona Hill, who also testified in the impeachment inquiry, that Giuliani and Sondland were trying to get Ukraine President Voldymyr Zelenskiy "to reopen investigations into Burisma."

Bolton resigned in September, and Morrison had similarly been expected to leave for some time. "I do not want anyone to think there is a connection between my testimony today and my pending departure," he wrote.

As a national security adviser, Morrison was among those listening to Trump's July 25 call with the Ukrainian leader that sparked a whistleblower's complaint and the impeachment inquiry.

He said he asked NSC lawyers to review the call because he had three concerns if word of the discussion leaked: how it would play out in polarized Washington, how it would affect bipartisan support in Congress for Ukraine and how it would affect U.S.-Ukraine relations.

Republican lawmakers portrayed the opening remarks of the longtime GOP policy operative as shifting the debate favourably toward Trump. They said Morrison's opening statement contradicted other witnesses, but they did not provide details.

"It's a very compelling witness today that is giving testimony that contradicts some of the testimony we heard," said Rep. Mark Meadows, R-N.C.

Another Republican, Rep. Chip Roy of Texas, said, "When you all see what he had to say, it will be interesting."

Democrats, though, have said the witnesses are largely corroborating the central argument of the impeachment inquiry -- that aid to Ukraine was being withheld as the Trump administration pushed the young democracy for the political investigation.

It is against the law to seek or receive assistance of value from a foreign entity in a U.S. election. Trump says he did nothing wrong.

Morrison had been featured prominently in previous testimony from Taylor, the top diplomat in Ukraine who testified before House investigators last week.

It was Morrison who first alerted Taylor to concerns over Trump's phone call with the Ukraine president.

In fact, Morrison's name appeared more than a dozen times in testimony by Taylor, who told impeachment investigators that Trump was withholding military aid unless Zelenskiy went public with a promise to investigate Biden and Burisma, where Biden's son served on the board.

Taylor's testimony contradicted Trump's repeated denials that there was any quid pro quo.

Morrison testified Thursday that he initially knew so little about Burisma when he took over for Hill in Julythat he had to do a Google search but quickly understood the Biden connection.

He did clarify one difference from Taylor's recollection of events: He said it was his understanding that "it could be sufficient" if the new Ukraine prosecutor general, rather than Zelenskiy himself, committed "to pursue the Burisma investigation."

As the security funds for Ukraine were being withheld, Morrison told the diplomat, "President doesn't want to provide any assistance at all."

Their concerns deepened when Morrison relayed on Sept. 7 the conversation he had with Sondland that gave him that "sinking feeling."

In it, Sondland explained that Trump said he was not asking for a quid pro quo but insisted that Zelenskiy "go to a microphone and say he is opening investigations of Biden and 2016 election interference," Taylor testified last week.

Morrison told Bolton and the NSC lawyers of this call between Trump and Sondland, according to Taylor's testimony.

The testimony came as the House took its first formal vote on the impeachment inquiry Thursday, approving the process ahead for public hearings and possible drafting of articles of impeachment.

The 232-196 tally split along partisan lines, with all but two voting Democrats supporting the package and all voting Republicans opposed. One Republican-turned-independent joined Democrats in approving the package.

Democrats said they will largely follow rules used during the impeachment proceedings of Presidents Richard Nixon and Bill Clinton. Trump and Republicans dismiss the process as a sham, and the president has directed his staff not to testify in the House inquiry.

"This is a very solemn day in the history of the country when the president's misconduct has compelled us to move forward with an impeachment inquiry," said Rep. Adam Schiff, D-Calif., the chairman of the Intelligence Committee leading the probe.

The spotlight has been on Morrison since August, when a government whistleblower said multiple U.S. officials had said Trump was "using the power of his office to solicit interference from a foreign country in the 2020 U.S. election."

Morrison, formerly a longtime Republican staffer at the House Armed Services Committee, has been bouncing around Washington in GOP positions for two decades.

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Associated Press writers Mike Balsamo, Mary Clare Jalonick, Alan Fram and Matthew Lee contributed to this report.