The federal Office of Government Ethics has accused Senate Republicans of speeding confirmation hearings for Donald Trump’s cabinet nominees.

OGE director Walter Shaub Jr. told top Senate Democrats in a Saturday letter that the stacked hearing schedule has left his office unable to complete ethics reviews on several nominees, which he called a matter of “great concern.”

“This schedule has created undue pressure on OGE’s staff and agency ethics officials to rush through these important reviews,” Shaub wrote to Senate Minority Leader Chuck Schumer (D-NY) and Sen. Elizabeth Warren (D-MA).

Here is the full @OfficeGovEthics letter to senators. The ending is ominous: “For as long as I remain director…” pic.twitter.com/daItfYjkh7 — Tamara Keith (@tamarakeithNPR) January 7, 2017

Shaub said that the incomplete reviews mean that some nominees could have “potentially unknown or unresolved ethics issues” that may not be addressed in the hearings slated to begin Tuesday.

“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 wrote.

The Senate will hold hearings for a number of Trump’s cabinet appointees next week, with six scheduled for Wednesday alone. Both billionaire charter school advocate Betsy DeVos, Trump’s pick for education secretary, and Exxon Mobil CEO Rex Tillerson, his choice for secretary of state, have hearings on Wednesday. These particularly controversial nominees have complex business ties, further complicating vetting efforts.

Schumer said that the OGE letter proved that Senate Republicans were trying to “jam through” nominees.

“The Office of Government Ethics letter makes crystal-clear that the transition team’s collusion with Senate Republicans to jam through these Cabinet nominees before they’ve been thoroughly vetted is unprecedented,” he said in a Saturday statement obtained by ABC News.

The Democratic National Committee asked GOP lawmakers to delay some of the hearings, writing in a Saturday statement that Senate Republicans would do so unless “they are concerned about what will be exposed” in a full ethics review.