Republicans slam Trump's decision on US forces in Syria that puts allies at risk "This, to me, is just unnerving to its core," one GOP senator said.

President Donald Trump’s decision to have U.S. forces in northern Syria step aside to make way for Turkish troops and put U.S.-allied Syrian Kurds at risk, has sparked outrage among Trump’s closest allies on Capitol Hill, with many Republicans questioning Trump’s leadership on foreign policy issues.

"I hope I'm making myself clear how shortsighted and irresponsible this decision is in my view," GOP Sen. Lindsey Graham of South Carolina, usually a strong Trump supporter, said Monday during an interview with “Fox & Friends.” "This to me is just unnerving to its core."

At the same time, Senate Majority Leader Mitch McConnell pointedly warned Trump to “exercise American leadership.”

Trump responded to the criticisms late Monday afternoon at the White House, and said he has "great respect" for the Republican leaders who opposed his decision.

"I have great respect for all of the people that you named, and they have their opinion and a lot of people do, and I could also name many more than you just named of people that [are] totally supportive," Trump said in response to a reporter's question. "You see the names coming out, people are extremely thrilled because they say it's time to bring our people back home."

"We're not a police force," he added.

"But many people agree with it very strongly and I understand both sides of that," he said. "I fully understand both sides to it."

Turkey is set to invade Syria and the U.S. said it would remove all of its forces from the "immediate area," according to the White House.

A U.S. official confirmed Monday that American troops in northeast Syria were pulling back.

The Turkish move is likely to put the Kurdish forces that helped the U.S. defeat ISIS in peril. Turkey considers the Kurdish forces -- the PKK and YPG -- terrorist organizations.

The news came late Sunday after President Trump spoke to Turkish President Recep Tayyip Erdogan by phone earlier in the day.

Graham said he’d be introducing a Senate resolution opposing the move.

“Just spoke to Sen @ChrisVanHollen about situation in Syria. We will introduce bipartisan sanctions against Turkey if they invade Syria and will call for their suspension from NATO if they attack Kurdish forces who assisted the U.S. in the destruction of the ISIS Caliphate,” he tweeted Monday.

"Hope and expect sanctions against Turkey - if necessary - would be veto-proof. This decision to abandon our Kurdish allies and turn Syria over to Russia, Iran, & Turkey will put every radical Islamist on steroids. Shot in the arm to the bad guys. Devastating for the good guys,” Graham added.

McConnell, in urging Trump to show "American leadership," said the U.S. must prevent conflict between Turkey and the Syrian Kurds.

“I urge the President to exercise American leadership to keep together our multinational coalition to defeat ISIS and prevent significant conflict between our NATO ally Turkey and our local Syrian counterterrorism partners. Major new conflict between Turkey and our partners in Syria would seriously risk damaging Turkey’s ties to the United States and causing greater isolation for Turkey on the world stage,” McConnell said in a statement Monday.

“This is a terribly unwise decision by the President to abandon our Kurdish allies, who have been our major partner in the fight against the Islamic State,” GOP Sen. Susan Collins of Maine said in a statement.

Sen. Ben Sasse, R-Neb., said Trump’s decision will result in the “slaughter” of allies.

“If the President sticks with this retreat, he needs to know that this bad decision will likely result in the slaughter of allies who fought with us, including women and children. I hope the President will listen to his generals and reconsider. And before Turkey butchers innocent Kurds, Erdogan should carefully consider his privileged status as a NATO member. The American people don’t partner with genocidal regimes,” Sasse said in a statement.

Sens. Mitt Romney, R-Utah, and Chris Murphy, D-Conn., said they plan to call administration officials to Capitol Hill to testify about the president''s decision.

“Barring a reversal of this decision, the Administration must come before the Senate Foreign Relations Committee and explain to the American people how betraying an ally and ceding influence to terrorists and adversaries is not disastrous for our national security interests. As Chairman and Ranking Member of the Middle East subcommittee, we will be working with Committee leadership to assure that the Administration appears before the Committee as soon as possible,” the senators said in a joint statement.