Washington (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.