The Trump administration has been found to have broken federal law by freezing millions of dollars of congressionally approved aid to Ukraine last year.

That’s according to a top government watchdog which deemed the White House had no right to withhold $US250 million ($361 million) in security assistance to Ukraine – the issue at the centre of Donald Trump’s impeachment trial.

The Government Accountability Office (GAO) said the Ukraine funding freeze ordered just hours after Mr Trump’s infamous call with then-president-elect Volodymyr Zelensky had violated the so-called Impoundment Control Act.

The Office of Management and Budget (OMB) “withheld funds for a policy reason, which is not permitted”, the independent watchdog said, arguing the money was withheld to advance the president’s own agenda. 

“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 report said.

Democrats are seizing on the GAO report as reinforcement of their argument that Mr Trump abused his authority for politically motivated purposes.

Issued just hours ahead of the start of the impeachment trial in the US Senate, the report couldn’t have come at a more inconvenient time for Republicans.

The impeachment charges are being read by lead House manager Rep Adam Schiff D-CA from the Well of the Senate pic.twitter.com/OZRevpIaBR

— Jamie Dupree (@jamiedupree) January 16, 2020

Under the US Constitution, Congress is granted the “power of the purse”, and the president, as the executive branch, is given little leeway in ignoring appropriations mandated by lawmakers and passed into law.

The OMB disagreed with the report’s conclusions.

“OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the President’s priorities and with the law,” spokesperson Rachel Semmel said.

It’s yet another blow to the White House’s efforts to justify Mr Trump’s actions last year and comes after Lev Parnas, a close associate of Mr Trump’s personal lawyer, made a series of potentially explosive claims about his pressure campaign in Ukraine.

In an exclusive interview aired on Thursday (local time), Mr Parnas said he delivered an ultimatum in May to the incoming president of Ukraine that no senior US officials would attend his inauguration and all American aid to the war-torn country would be withheld if an investigation into Joe Biden wasn’t announced.

The next day, the US State Department announced that Vice President Mike Pence would no longer be attending Mr Zelenskiy’s inauguration.

Mr Trump ordered him to stay away so that Ukraine would take seriously the demand for an investigation into the Democratic presidential candidate, Mr Parnas alleged.

If true, Mr Parnas’ account undercuts a key Republican defence of Mr Trump deployed during the ongoing impeachment fight – that Mr Trump’s withholding of vital military aid to Ukraine last summer wasn’t a quid pro quo for Biden investigations because Mr Zelenskiy didn’t know the money was being held up.

Mr Trump was impeached by the US House of Representatives on December 18.

The articles of impeachment against Mr Trump before an engrossment ceremony on January 15. Photo: Getty

Later on Thursday, seven managers appointed by the House – effectively the prosecutors –  are due to appear in the Senate and read out the two articles of impeachment to the upper chamber.

Chief Justice John Roberts is later expected to begin swearing in the 100 senators for the trial.

The House managers face an uphill battle in the Republican-controlled Senate, where Mr Trump’s conviction and removal from office would require a two-thirds majority.

-with AAP