The White House budget office is further defending its withholding of security assistance to Ukraine, detailing in a new letter that it temporarily paused distribution of the funds to ensure the money was not spent prematurely in a way that was at odds with the Trump administration's foreign policy.

Obtained by CBS News, the memo from Mark Paoletta of the Office of Management and Budget (OMB) to the Government Accountability Office sent Wednesday describes the freeze on the money to Ukraine as a "programmatic delay," under which the administration conducts a process to "determine the best policy for the efficient and effective use of funds consistent with the intent of the statute."

"OMB took appropriate action, in light of a pending policy process, to ensure that funds were not obligated prematurely in a manner that could conflict with the president's foreign policy," Paoletta wrote.

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The Washington Post first reported the details of the memo.

The security assistance withheld from Ukraine is at the center of House Democrats' impeachment investigation into President Trump, which entered a new phase this week after the House Judiciary Committee introduced two articles of impeachment against the president. The committee is debating the articles Thursday, and is expected to vote on them later in the day.

One article charges Mr. Trump abused his powers by conditioning the release of aid to Ukraine on a public announcement from Ukrainian President Volodymyr Zelensky of an investigation of Vice President Joe Biden. The other accuses the president of obstructing Congress by directing administration officials not to comply with congressional subpoenas.

The president and his allies have argued he wanted to withhold the assistance due to concerns of corruption in Ukraine and to ensure other European nations were contributing their own aid.

According to Paoletta's memo, the budget office first learned Mr. Trump had questions about the Defense Department's plans for the security assistance to Ukraine on June 19. OMB placed a hold on the money in response to an administration directive that it not be spent "pending a policy decision" on July 25 — the day of the president's call with Zelensky. That freeze was extended eight times across August and September before it was lifted September 12, the memo states.

The budget office stressed that during the pause, Defense Department lawyers never said it would not be able to obligate the money before the end of the fiscal year on September 30.

"When the executive branch is executing the laws, it is routinely necessary to reassess policy goals based on program effectiveness and other factors," the memo states. "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."

In detailing the budget office's legal rationale for withholding the security assistance to Ukraine, Paoletta wrote the "president has an obligation to ensure that funds are being spent prudently," and "the executive branch has a duty to taxpayers to ensure that appropriations are spent wisely, in accordance with statutory requirements."

The withholding of the Ukrainian assistance caused consternation within OMB, and at least one attorney with the budget office resigned in part over the freeze, Mark Sandy, an OMB official, told House investigators last month.

Sandy told lawmakers he raised concerns about the legality of the delay in military aid, and said it wasn't until early September that Mr. Trump asked OMB for information about contributions to Ukraine from other countries.

He further told investigators the hold was unusual and could not "recall another event like it."