A top adviser to US President Donald Trump on Ukraine has testified that after listening to Mr Trump ask Ukraine's president to investigate Joe Biden, he was so alarmed that he reported the matter to a White House lawyer out of concern for US national security.

Army Lieutenant Colonel Alexander Vindman, director of European affairs on the National Security Council, is the first current White House official to testify in the House of Representatives impeachment inquiry against Mr Trump.

Mr Vindman, a Ukraine-born US citizen and decorated Iraq War combat veteran, also became the first person to testify who listened in on the 25 July call at the heart of the Ukraine scandal.

Even before his arrival, some allies of the Republican president, including Fox News host Laura Ingraham, sought to attack Lt Col Vindman's integrity and questioned his loyalty to the United States.

"I was concerned by the call," Lt Col Vindman said in his opening statement to the three House committees conducting the Democratic-led impeachment inquiry.

"I did not think it was proper to demand that a foreign government investigate a US citizen, and I was worried about the implications for the US government's support of Ukraine."During the call, Mr Trump asked Ukrainian President Volodymyr Zelenskiy to investigate Mr Biden, a Democratic political rival, and his son Hunter Biden, who had served on the board of Ukrainian gas company Burisma.

Mr Trump also asked Mr Zelenskiy to investigate a debunked conspiracy theory that Ukraine, not Russia, interfered in the 2016 US election.

Mr Trump had withheld $391 million in US security aid to Ukraine approved by Congress to fight Russia-backed separatists in the eastern part of the country. Mr Zelenskiy agreed to Mr Trump's requests. The aid was later provided.

Lt Col Vindman, who appeared after receiving a subpoena from lawmakers, recounted listening in on the call.

"I realised that if Ukraine pursued an investigation into the Bidens and Burisma, it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained. This would all undermine US national security," he said in his testimony.

After the call, Lt Col Vindman added, he reported his concerns to the National Security Counsel's lead counsel.

The call prompted a complaint from an intelligence community whistleblower, whose identity has not been revealed, that triggered the impeachment inquiry. In his statement, Lt Col Vindman denied being the whistleblower or knowing the identity of the individual.

At a 10 July meeting in Washington with visiting Ukrainian officials, Lt Col Vindman said US Ambassador to the European Union Gordon Sondland, a former Trump political donor, told the Ukrainian officials they needed to "deliver specific investigations in order to secure a meeting with the president."

At that point, Lt Col Vindman said, then-National Security Adviser John Bolton cut the meeting short.

According to Lt Col Vindman's prepared remarks, Mr Sondland told other US officials in a debriefing after the meeting that it was important that the Ukrainian investigations centre on the 2016 election, the Bidens and Burisma.

US Ambassador to the EU Gordon Sondland

"I stated to Amb Sondland that his statements were inappropriate, that the request to investigate Biden and his son had nothing to do with national security," Lt Col Vindman said, adding that he also reported his concerns to the National Security Counsel's lead lawyer.

Mr Trump's former Russia adviser, Fiona Hill, testified in the impeachment inquiry on 14 October that she too was alarmed by Mr Sondland's reference to a probe of Biden during that 10 July meeting and was advised to see NSC lawyer John Eisenberg, a person familiar with her testimony told Reuters.

Mr Sondland gave a different account of the 10 July events in his own testimony in the inquiry, saying that "if Ambassador Bolton, Dr Hill or others harbored any misgivings about the propriety of what we were doing, they never shared those misgivings with me, then or later."

Democrats have accused Mr Trump of pressuring a vulnerable foreign ally to interfere in a US election for his own political benefit. Federal law prohibits candidates from accepting foreign help in an election.

Mr Biden is a leading contender for the Democratic presidential nomination to face Mr Trump in the November 2020 election.

It comes after US Democrats announced that the House of Representatives will hold its first formal vote on Thursday on the impeachment inquiry into President Trump.

In a letter to her fellow Democrats, Speaker of the House Nancy Pelosi said the resolution will spell out the next steps in the inquiry, which is expected to include public hearings.

Up to now there has been no formal impeachment vote in the US House of Representatives and the impeachment inquiry has been held behind closed doors.

President Trump and his fellow Republicans have criticised Democrats for holding their hearings in private, saying the inquiry was invalid.

Last night it was announced that a vote will take place on Thursday which will lay the groundwork for the next phase of the impeachment inquiry which is expected to include public hearings.

The White House said it was an admission Ms Pelosi that Democrats were conducting an unauthorised impeachment proceeding.

Additional reporting Brian O'Donovan