WASHINGTON (AP) — The federal government's watchdog agency said Thursday a White House office violated federal law in withholding security assistance to Ukraine.

The Government Accountability Office said in a report that the Office of Management and Budget violated the law in holding up the aid. The freeze is at center of the impeachment of President Donald Trump.

The independent agency, which reports to Congress, said OMB violated the Impoundment Control Act in delaying the security assistance Congress authorized for Ukraine 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, Thomas Armstrong, in the report.

OMB has argued the hold was appropriate and necessary.

"We disagree with GAO's opinion. 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 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. The Senate is set to begin its trial on Thursday.