Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellFEC flags McConnell campaign over suspected accounting errors Poll: 59 percent think president elected in November should name next Supreme Court justice Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ky.) said he was "distressed" by Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE's resignation announcement Thursday.

“I believe it’s essential that the United States maintain and strengthen the post-World War II alliances that have been carefully built by leaders in both parties," McConnell said in a statement. "We must also maintain a clear-eyed understanding of our friends and foes, and recognize that nations like Russia are among the latter."

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“So I was sorry to learn that Secretary Mattis, who shares those clear principles, will soon depart the administration. But I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership," McConnell said.

Thursday's statement is one of McConnell's more critical statements toward Trump since the president's inauguration.

For his resignation, Mattis sent a letter to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE saying he deserved a secretary "whose views are better aligned with yours."

Mattie's announcement that he will resign in February came a day after the Trump administration announced it would be withdrawing the U.S. military presence from Syria and amid reports that the same may happen in Afghanistan.

Both of those moves have been met with opposition from Pentagon officials and members of the foreign policy establishment.

In his note, Mattis signaled concern with the way the president treated allies in NATO as well as rivals such as China and Russia.

McConnell finished his statement by asking Trump to consider a replacement whose views align with Mattis's.

“It is regrettable that the president must now choose a new Secretary of Defense," McConnell wrote. "But I urge him to select a leader who shares Secretary Mattis’s understanding of these vital principles and his total commitment to America’s servicemembers."