A decision by the Trump administration to withhold military aid to Ukraine was a violation of the law, a key independent US government watchdog agency has found.
The Office of Management and Budget "withheld funds for a policy reason, which is not permitted" by law, the US Government Accountability Office (GAO) said in its report.
Thursday's report was issued just hours ahead of the start of the impeachment trial in the US Senate and is another blow to the White House's efforts to justify President Donald Trump's actions last year.
"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 said, noting that lawmakers legally appropriated $US250 million ($A361 million) dollars for Ukraine in the 2019 fiscal year.
Trump was impeached by the US House of Representatives on December 18.
He was accused of pressuring Ukraine to announce an investigation into Joe Biden - a leading candidate to run against Trump in elections this year - by withholding critical military aid and a White House meeting.
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.
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 Trump's conviction and removal from office would require a two-thirds majority.