WASHINGTON — The federal government's watchdog agency says the White House office violated federal law in withholding security assistance to Ukraine aid. 

The Government Accountability Office said the White House Office of Management and Budget violated the law in holding up the assistance. 

In a statement issued Thursday, the Government Accountability Office said that when the Office of Management and Budget withheld funds appropriated to the Department of Defense for security assistance to Ukraine, it violated the Impoundment Control Act because funds were held for policy reasons.

"Faithful execution of the law does not permit the President to substitute his own police priorities for those that Congress has enacted into law. [The Office of Management and Budget] withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA). The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA," the U.S. Government Accountability Office's statement said.

The freeze is at center of the impeachment of President Donald Trump. 

Trump was impeached last month on charges of abusing his power for pressuring Ukraine to investigate Democratic rivals, as he was withholding the aid, and for obstructing Congress' ensuing probe. 

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The budget office said it disagrees with the conclusion and that the hold on aid was appropriate.

The statement comes Thursday, the same day senators and the Chief Justice of the Supreme Court are scheduled to take their oaths for Trump's impeachment trial.

According to the Associated Press, the seven impeachment managers who will present the prosecution -- all House Democrats -- will go to the Senate at noon EST to formally present the articles. The managers delivered the articles Wednesday in a traditional, ceremonial procession.