Just as Cabinet nominees prepare for their confirmation hearings before the Senate next week, a federal ethics watchdog agency is expressing concerns over what it called an unusually rushed process of vetting President-elect Donald Trump’s Cabinet picks.

In a letter sent to Senate Democrats Saturday, the director of the Office of Government Ethics, Walter Shaub Jr., said the schedule of Cabinet hearings created “undue pressure on OGE’s staff and agency ethics officials to rush through these important reviews.”

“It has left some of the nominees with potentially unknown or unresolved ethics issues shortly before their scheduled hearings,” Shaub wrote in the letter obtained by CBS News. It was of further “great concern” to Shaub that several of Mr. Trump’s Cabinet selections had yet to complete the office’s full ethics review. For some nominees, the office had not received “even initial draft financial disclosure reports for some of the nominees scheduled for hearings,” according to the ethics director.

“I am not aware of any occasion in the four decades since OGE was established when the Senate held a confirmation hearing before the nominee had completed the ethics review process,” he said in the letter, addressed to Senate Minority Leader Chuck Schumer of New York and Sen. Elizabeth Warren of Massachusetts. The letter, written in response to questions by Senate Democrats on the issue, was also sent to Senate Majority Leader Mitch McConnell.

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On Saturday afternoon, the Trump transition team pushed back on the concerns expressed in the letter, calling it a “disservice to the country” from the ethics office.

“President-elect Trump is putting together the most qualified administration in history and the transition process is currently running smoothly,” a transition spokesperson said in a statement to CBS News. “In the midst of a historic election where Americans voted to drain the swamp, it is disappointing some have chosen to politicize the process in order to distract from important issues facing our country.”

Over the next week, at least seven Cabinet picks are scheduled to appear before Senate committees, with three scheduled for Wednesday: Mr. Trump’s education chief pick Betsy DeVos, his transportation secretary choice Elaine Chao, and one of his most controversial appointments, Rex Tillerson, a billionaire oil executive with close business ties to Russia, for secretary of state.

But the letter from the Office of Government Ethics, which oversees problems with the executive branch’s potential conflicts of interests, could present a roadblock for Senate Republicans hoping for speedy confirmations.

Shaub warned that it would be “cause for alarm if the Senate were to go forward with hearings on nominees whose reports OGE has not certified.”

“For as long as I remain Director, OGE’s staff and agency ethics officials will not succumb to pressure to cut corners and ignore conflicts of interest,” he said.

Schumer, the new Democratic leader in the Senate, urged Republicans in the upper chamber to “heed the advice of this independent office and stop trying to jam through unvetted nominees.”

The letter “makes crystal clear,” Schumer said in a statement Saturday, that the process to get Cabinet nominees pushed through before they’ve been reviewed by the ethics office is “unprecedented.”

In her own tweet Saturday, Warren also called on the Senate to delay confirmation hearings until ethics reviews were completed, saying it was “ridiculous” for nominees to “drag their feet on ethics paperwork.”

This is ridiculous. @realDonaldTrump’s noms can’t drag their feet on ethics paperwork while their Senate friends try to run out the clock. — Elizabeth Warren (@SenWarren) January 7, 2017

Cabinet officials must put our country's interests before their own. No conf hearings should be held until we’re certain that’s the case. — Elizabeth Warren (@SenWarren) January 7, 2017

McConnell’s office did not immediately respond to a request for comment.