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(CNN) —  

Nikki Haley, President Donald Trump’s former ambassador to the United Nations, issued stinging criticism of her former boss on Monday, saying Trump’s decision to remove US troops from northern Syria as Turkey plans a military offensive in the region means the US is leaving Kurdish allies “to die.”

“We must always have the backs of our allies, if we expect them to have our back. The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake. #TurkeyIsNotOurFriend,” Haley, who also served as governor of South Carolina, wrote in a tweet.

Responding to the former ambassador, the Syrian Democratic Forces said in its own tweet that “The world relies on strong #American leaders like (Haley)” and that it hopes “our US partners will reverse this decision & not abandon our combined effort in NE Syria. #TurkeyIsNotOurFriend.”

The White House announced Trump’s decision Sunday night following a phone call between Trump and Turkish President Recep Tayyip Erdogan. The move marks a major shift in US foreign policy and effectively gives Turkey the green light to attack US-backed Kurdish forces. The group, long considered as among Washington’s most reliable partners in Syria, has played a key strategic role in the campaign against ISIS in the region.

Haley, who left the administration late last year, is not known for publicly dissenting with the President. In December, after announcing she planned to leave her post, she told NBC she used Trump’s “unpredictable” behavior to her advantage in order to “get the job done” as ambassador.

The President’s Syria decision was also slammed by Brett McGurk, the former US envoy for the global coalition to counter ISIS who resigned last year because of Trump’s decision to pull US forces out of Syria – a decision the President later reversed.

“Donald Trump is not a Commander-in-Chief. He makes impulsive decisions with no knowledge or deliberation. He sends military personnel into harm’s way with no backing. He blusters and then leaves our allies exposed when adversaries call his bluff or he confronts a hard phone call,” McGurk, who also served in the Obama administration, wrote in a tweet.

The former diplomat said in another tweet that Trump’s decision “demonstrates a complete lack of understanding of anything happening on the ground” and added in a separate tweet that it was “malpractice.”

“Bottom line: Trump tonight after one call with a foreign leader provided a gift to Russia, Iran, and ISIS,” McGurk wrote.

Republican Sens. Mitt Romney of Utah and Lindsey Graham of South Carolina also sharply criticized Trump’s decision, with Romney, who Trump attacked on Twitter last weekend, saying the move “is a betrayal” to the US’ Kurdish allies in Syria.

“It says that America is an unreliable ally; it facilitates ISIS resurgence; and it presages another humanitarian disaster,” Romney wrote.

Graham, who is a steadfast supporter of Trump, said in an interview with Fox News that the decision was “shortsighted and irresponsible,” and wrote in a tweet that he would introduce a Senate resolution opposing it if Trump doesn’t reverse course.

Later on Monday Graham tweeted that he and Democrat Sen. Chris Van Hollen of Maryland will introduce bipartisan sanctions to call for Turkey to be suspended from NATO if it attacks Kurdish forces.

CNN’s Helen Regan, Jeremy Diamond and Ryan Browne contributed to this report.