WASHINGTON — Top allies of Donald Trump castigated the US president Monday, after his surprise announcement that US forces would withdraw from Syria and allow a Turkish offensive against US-allied Kurds in the country, saying the move would enable Iran to gain a greater foothold in Syria and render Israel militarily vulnerable.

“The most probable outcome of this impulsive decision is to ensure Iran’s domination of Syria,” tweeted South Carolina Senator Lindsey Graham, a Republican. “The U.S. now has no leverage and Syria will eventually become a nightmare for Israel.”

Former US envoy to the United Nations Nikki Haley said Trump, her old boss, was abandoning a vital American ally.

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“We must always have the backs of our allies, if we expect them to have our back,” she tweeted. “The Kurds were instrumental in our successful fight against ISIS in Syria. Leaving them to die is a big mistake.”

On Sunday night, the White House said US forces in northeastern Syria will step aside and clear the way for an expected Turkish assault — essentially abandoning Kurdish fighters who fought alongside American forces in the years-long battle to defeat the Islamic State group.

Turkish President Recep Tayyip Erdogan has threatened for months to launch a military operation across the border. He views the Syria Kurdish forces as a threat to his country as Ankara has struggled with a Kurdish insurgency within Turkey.

Republicans and Democrats have long warned that allowing a Turkish attack could lead to a massacre of the Kurds and send a troubling message to American allies across the globe.

Several of Trump’s closest allies on Capitol Hill voiced similar concerns Monday.

“If reports about US retreat in #Syria are accurate, the Trump administration has made a grave mistake that will have implications far beyond Syria,” tweeted Florida Senator Marco Rubio, a Republican. “It would confirm #Iran’s view of this administration & embolden then to escalate hostile attacks which in turn could trigger much broader & more dangerous regional war.”

Graham said that he consulted with Senator Chris Van Hollen, a Democrat from Maryland, and would introduce bipartisan legislation to sanction Turkey if it attacks Syria — and call for Ankara’s suspension from the North Atlantic Treaty Organization.

“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,” Graham tweeted.

Trump defended his decision, saying that he “was elected on getting out of these ridiculous endless wars.” If Turkey were to do anything “off limits,” he said, he would consequence the country harshly.

“As I have stated strongly before, and just to reiterate, if Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the Economy of Turkey (I’ve done before!),” he tweeted. “They must, with Europe and others, watch over the captured ISIS fighters and families.”

“It is time now for others in the region, some of great wealth, to protect their own territory,” he added. “THE USA IS GREAT!”

The Associated Press contributed to this report.