WASHINGTON (BLOOMBERG) - The White House budget office violated federal law when it withheld about US$214 million (S$288 million) appropriated by Congress to the Defence Department for security aid to Ukraine, an independent congressional watchdog agency concluded.

"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 nonpartisan Government Accountability Office (GAO) said in a report released on Thursday (Jan 16).

The Office of Management and Budget "withheld funds for a policy reason, which is not permitted under the Impoundment Control Act."

The delay in delivering security aid to Ukraine last summer is central to the impeachment case against President Donald Trump brought by the House.

The impeachment articles charge that Trump withheld the aid, and a White House visit by Ukraine's new president, in order to force the government there to announce an investigation that would entangle one of Trump's chief political rivals, former vice-president Joe Biden. They also accuse Trump of blocking documents and testimony about the Ukraine aid.

Democrats said the report bolsters their argument that when Trump's trial in the Senate gets under way, Republicans should allow introduction of witnesses and evidence that weren't available to House investigators.

"The GAO's independent findings reinforce the need for the Senate to obtain all relevant documents and hear from key fact witnesses in order to have a fair trial," Democratic Senator Chris Van Hollen of Maryland, who requested the GAO investigation in December, said in a statement.

Russ Vought, acting director of the OMB, dismissed the GAO's conclusions and said on Twitter that the administration "complied with the law at every step."

Vought is one of the administration officials who declined to testify before the House impeachment inquiry.

The GAO report was released hours before senators were set to be sworn in as jurors in Trump's impeachment trial. Several Republican senators said it didn't change how they view the need for presentation of additional evidence.

Under the 1974 Impoundment Control Act, the executive branch is legally obligated to spend the money that Congress approves and which the president signs into law.

The aid was held up at the same time Trump's personal lawyer, Rudy Giuliani was making a backchannel approach to Ukraine as part of an effort to get the government there to announce an investigation into Biden and his son, Hunter Biden, who had been on the board of a Ukrainian energy company, according to testimony in the House impeachment inquiry.

The aid was aimed at helping Ukraine fend off Russian-backed separatists. It was eventually released in September - after members of Congress began questioning the delay and a whistle-blower raised an alarm about Trump's phone call with Ukraine's President Volodymyr Zelenskiy.

"The aid got out with no detriment to our national security," Defence Department spokesman Jonathan Hoffman told reporters at the Pentagon on Thursday.

The GAO report contends that "There was no external factor causing an unavoidable delay. Rather, OMB on its own volition explicitly barred DOD from obligating amounts."

In a letter sent to the GAO on Dec 11, OMB General Counsel Mark Paoletta wrote that the delay in obligating funds was a "programmatic delay" and not a "deferral" and thus permissible under the law.

"When the Executive branch is executing the laws, it is routinely necessary to reassess policy goals based on program effectiveness and other factors," Paoletta wrote. "Ensuring that there is time to conduct a meaningful process that results in successful policies (and that funds are not used in opposition to such policies in the meantime) often requires that obligations pause until that policy process is concluded."

"GAO has recognised that policy considerations can equate to programmatic delay," Paoletta wrote.

Democratic Senators Dick Durbin of Illinois and Debbie Stabenow of Michigan said in interviews that the GAO report adds to their push to bring in acting White House Chief of Staff Mick Mulvaney, OMB officials and documents discussing the holdup in Ukraine aid at the heart of the impeachment battle.

"How many times have we heard Republicans say 'There's no crime here, no law broken.' Now the Government Accountability Office has said otherwise," Durbin said.

Democrats also want testimony from former Trump national security adviser John Bolton, who's said he'd be willing to appear under a subpoena from the Senate.

Democrats would need to persuade at least four Republicans to vote in favor of calling witnesses during the Senate trial. Senators Susan Collins, Lisa Murkowski, Mitt Romney and Lamar Alexander have said they're open to hearing new testimony, though they've not committed to calling any of those who Democrats are seeking to call.

Collins, Murkowski and Romney, along with several other GOP senators, said Thursday that they hadn't yet read the report.

GOP Senator Mike Rounds of South Dakota said "breezed through" the watchdog agency's report and noted that the GAO indicated that the administration can defer or hold aid, though Congress should have been notified.

"In the the future, I would expect that OMB would change it and would give the appropriate notices," Rounds said. It likely won't change how senators view the need for new evidence.

Republican Senator John Cornyn of Texas said the report focuses on the OMB, not Trump.

"The GAO report identifies the OMB, not the president, and said it was for policy reasons, not for political reasons," Cornyn said. "I think we're going to hear some more about it but I don't think that changes anything."