U.S. Sen. Cory Gardner said Monday that he’s “deeply concerned” about President Donald Trump’s surprise decision to pull U.S. troops out of northern Syria in a move numerous Trump loyalists warn could endanger a key ally in the region while strengthening the Islamic State.

“I am deeply concerned that this decision fails to protect allies who gave their lives alongside our troops to accomplish U.S. objectives and fails to facilitate our continued efforts against ISIS,” Gardner told Colorado Politics in a statement emailed Monday afternoon by a spokeswoman. “At a minimum, these concerns must be addressed and Congress must have a strong oversight role.”

Gardner, a member of the Senate Foreign Relations Committee, added his voice to a chorus of Republican lawmakers and conservative national security experts who denounced the White House’s decision in stronger terms, with some of Trump’s staunchest allies calling the president’s decision “a disaster,” “a big mistake” and “unnerving to its core.”

The policy shift, announced late Sunday night, immediately triggered criticism that withdrawing U.S. troops from the war-torn country would allow the Turkish military to massacre Kurdish fighters who have fought the Islamic State alongside American troops for years.

Nikki Haley — Trump’s former ambassador to the United Nations — described Trump’s decision as a betrayal of a U.S. ally.

“The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake,” tweeted Haley, who campaigned for Gardner in Colorado last month and headlined a Trump fundraiser with Gardner in New York this weekend.

Republican U.S. Sens. Lindsey Graham of South Carolina and Marco Rubio of Florida unloaded on the president in uncharacteristically harsh terms.

Graham called called the decision “a disaster in the making” that risked plunging the region into chaos.

“I hope I’m making myself clear how short-sighted and irresponsible this decision is,” Graham told Fox News, adding that the president’s move “is just unnerving to its core.”

Rubio tweed that Trump’s new policy is “a grave mistake that will have implications far beyond Syria.”

Earlier Monday, U.S. Rep. Jason Crow, a Colorado Democrat, issued a joint statement along with a bipartisan group of lawmakers condemning the president’s decision as “a misguided and catastrophic blow to our national security interests.”

The Associated Press contributed.