The White House has violated federal law in withholding security assistance to Ukraine, a United States federal watchdog agency has said.

Key points: The findings are not legally binding and have no prosecutorial power

The findings are not legally binding and have no prosecutorial power The aid was held up on orders from Mr Trump but was released after a complaint

The aid was held up on orders from Mr Trump but was released after a complaint Ukraine has asked the FBI for help in investigating a suspected cyberattack by Russia

The Government Accountability Office (GAO) said in a report that the Trump Administration violated the law in holding up the aid, which Congress passed less than a year ago.

"The President is not vested with the power to ignore or amend any such duly enacted law," the report said.

The aid in question was held up last summer on orders from US President Donald Trump but was released in September after a whistleblower's complaint became public. The action has become the centre of Mr Trump's impeachment.

The independent agency, which reports to Congress, said Office of Management and Budget (OMB) violated the Impoundment Control Act in delaying the security assistance for "policy reasons," rather than technical budgetary needs.

"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," wrote the agency's general counsel in the report.

Its findings are not legally binding, but its reports are seen by politicians as objective, reliable and generally uncontested. The GAO has no prosecutorial power.

Capitol Hill Democrats seized on the report as evidence of a lawless White House led by Acting Chief of Staff Mick Mulvaney, who is a key figure in the impeachment investigation of Mr Trump. He is still officially the OMB director.

"Congress makes funding decisions, and the Trump Administration's illegal impoundment of these vital national security funds was a brazen assault on the checks and balances inherent to our democracy," said House Appropriations Committee Chairwoman Nita Lowey.

"Given that this illegal conduct threatened our security and undermined our elections, I feel even more strongly that the House has chosen the right course by impeaching President Trump.

"No-one is above the law."

However, the OMB has argued the hold was appropriate and necessary.

"OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President's priorities and with the law," said OMB spokeswoman Rachel Semmel.

Trump's dealings with Ukraine

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 26 seconds 1 m 26 s US Democrats send impeachment articles to Senate

Mr Trump was impeached last month on charges of abusing his power.

The Democratic-led House of Representatives on Wednesday (local time) sent the Senate the two charges it passed accusing Mr Trump of abuse of power and obstruction of Congress arising from his dealings with Ukraine, clearing the way for only the third impeachment trial of a president to begin next week.

The abuse of power cited by the House included Trump's withholding of $US391 million ($567 million) passed by Congress in security aid for Ukraine, a move aimed at pressuring Kiev into investigating political rival Joe Biden, the Republican president's possible Democratic opponent in the upcoming US election.

"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the GAO concluded, referring to the fact that Congress had already voted to appropriate the funds.

While the agency's assessment was a setback to Mr Trump, it was unclear how or even if it would figure in his trial in the Republican-led Senate given that key issues such as whether witnesses will appear or new evidence will be considered remain up in the air.

House members voted largely along party lines on Wednesday (local time), to give the Senate the task of putting the Republican president on trial.

The Senate is expected to acquit Mr Trump, keeping him in office, as none of its 53 Republicans has voiced support for removing him, a step that requires a two-thirds majority.

Mr Trump denies wrongdoing and has called the impeachment process a sham.

Hacking attack by Russian special service: Ukraine

There has been no evidence of wrongdoing by Joe Biden, right, and his son Hunter, left. ( Reuters: Carlos Barria )

Ukraine has asked the FBI for help in investigating a suspected cyberattack by Russian military hackers on the energy company Burisma, a Ukrainian interior ministry official said.

At the same briefing, the interior ministry also announced an investigation into the possible illegal surveillance of the former American ambassador to Kiev, Marie Yovanovitch, following messages contained in documents released by the US Congress this week.

Burisma Holdings was at the centre of attempts by Mr Trump last July to persuade Ukrainian authorities to announce an investigation into Hunter Biden, son of Democratic US presidential contender Joe Biden.

California-based cybersecurity company Area 1 Security on Monday (local time) identified the hacking of Burisma and linked it to Russia's Main Directorate of Military Intelligence, or GRU.

The same hacking group, known as "Fancy Bear" or "APT28" by cybersecurity researchers, breached the Democratic National Committee in 2016 in what US investigators described as part of an operation to disrupt that year's election.

Russia's defence ministry did not respond to a request for comment about Area 1 Security's assertions.

"It is noted that the hacking attack was probably committed by the Russian special services," Ukrainian interior ministry official Artem Minyailo said at a briefing.

Mr Minyailo said Ukraine had asked the FBI and Area 1 Security for assistance regarding information that hackers stole personal employee data and emails from executives at Burisma and other companies.

These other companies included the media production company of President Volodymyr Zelenskiy, he said.

Hunter Biden used to have a seat on the board of Burisma. There has been no evidence of wrongdoing by the Bidens, who reject Mr Trump's allegations of corruption.

AP/Reuters