WASHINGTON ― Senate Majority Leader Mitch McConnell (R-Ky.) said he is “particularly distressed” over the resignation of Defense Secretary Jim Mattis, who announced Thursday he would be leaving early next year after irreconcilable policy differences with President Donald Trump.

“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. We must also maintain a clear-eyed understanding of our friends and foes, and recognize that nations like Russia are among the latter,” McConnell said in an unusually blunt statement on Thursday evening.

“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,” he added.

McConnell’s statement is a rare public expression of disagreement with Trump. The Senate GOP leader avoids critiquing the president and almost always declines to answer questions about Trump’s numerous controversial statements and blistering attacks against foes, including special counsel Robert Mueller.

It’s clear that the resignation of the retired Marine general, who was one of Trump’s first Cabinet picks, rattled even Trump’s staunchest defenders on Capitol Hill, however. What added to the consternation was the list of grievances Mattis detailed in a remarkable letter Thursday, in which he said he and Trump had differing views on countering Russian and Chinese authoritarianism, as well as the utility and importance of alliances across the globe.

“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mattis said in the letter.

Mattis was also reportedly incensed by Trump’s decision this week to unilaterally and abruptly withdraw U.S. troops from Syria, a move lawmakers fear will allow the self-described Islamic State to thrive and cause further instability in the region.

In his statement, McConnell urged Trump to nominate a replacement for Mattis who agrees with Mattis’ worldview.

“It is regrettable that the president must now choose a new Secretary of Defense. 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,” McConnell said.

While many Republican lawmakers on Capitol Hill echoed McConnell’s comments about Mattis’ resignation, expressing alarm about the direction of the administration’s foreign policy in the Middle East, retiring House Speaker Paul Ryan (R-Wis.) issued a statement that offered only praise for the Pentagon chief.

“The House is indebted to Secretary Mattis for his service,” Ryan said Thursday. “With our country’s defense in his hands, we have become a safer America at home, and a more commanding force abroad. His patriotism and grit are matched only by his humility and graciousness.”