WASHINGTON – Calling it a "stain on America's honor," Sen. Lindsey Graham on Monday denounced President Donald Trump's decision to pull U.S. troops out of northeastern Syria as Turkey prepares a military assault against Kurdish fighters who helped the U.S. battle the Islamic State.

Graham phoned into "Fox & Friends," which Trump is known to watch regularly, to express his displeasure at the "impulsive decision by the president," calling it "short-sighted and irresponsible." He said the move has "undone all the gains we've made" and "thrown the region into further chaos."

"This to me is just unnerving to its core," Graham said.

The Kurdish fighters,America's chief ally in battling Islamic State terrorists in Syria, said in a statement that their U.S. allies "did not fulfill their obligations," as U.S. troops began to withdraw from their positions ahead of the expected Turkish militaryassault.

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"A precipitous withdrawal of U.S. forces from Syria would only benefit Russia, Iran, and the Assad regime. And it would increase the risk that ISIS and other terrorist groups regroup," said Senate Majority Leader Mitch McConnell, R-Ky., in a statement.

"As we learned the hard way during the Obama Administration, American interests are best served by American leadership, not by retreat or withdrawal," McConnell said.

Secretary of State Mike Pompeo and other administration officials had promised the U.S. would ensure that "the Turks don't slaughter the Kurds." The Kurdish fighters in Syria are known as the Syrian Democratic Forces.

Turkey's President Recep Tayyip Erdogan considers the Kurdish forces in Syria to be terrorists allied with Kurdish insurgents within his country and has long threatened a military incursion into the area.

Foreign policy experts, as well as Republican and Democratic lawmakers, have warned that allowing Turkey into the region could lead to a massacre of the Kurds and would be seen asan American betrayal of a vital military ally.

President Donald Trump defended the move on Twitter Monday, writing the U.S. "was supposed to be in Syria for 30 days" and "that was many years ago."

"We stayed and got deeper and deeper into battle with no aim in sight," Trump tweeted.

"The Kurds fought with us, but were paid massive amounts of money and equipment to do so," Trump added. He repeated his aversion to long-term U.S. military commitments overseas and said, "WE WILL FIGHT WHERE IT IS TO OUR BENEFIT, AND ONLY FIGHT TO WIN."

"Turkey, Europe, Syria, Iran, Iraq, Russia and the Kurds will now have to figure the situation out," he added.

In a later tweet, Trump threatened to "destroy" Turkey's economy if it does something he considers "off limits."

"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!)," the president tweeted.

Graham said he and Sen. Chris Van Hollen, D-Md., planned to introduce bipartisan sanctions on Turkey if they invade Syria and to call for Turkey's suspension from NATO if they attack the Kurds. He said he hoped the legislation would have a veto-proof majority.

The White House said Erdogan told Trump of his plans to move ahead with a military incursion into Syria during a phone call on Sunday night.

A senior State Department official, who briefed reporters on the condition of anonymity, said the president made it clear in that phone call that the United States does not support Turkey’s plans to invade Syria “in any way shape or form.”

This official said Trump’s decision to withdraw two military units from the border was based on two goals: The president does not want U.S. forces to be seen as greenlighting Turkey’s plans, nor does he not want Americans in the line of fire if Erdogan goes through with his threat.

“We don’t want to ever signal a military action that the president has not expressly ordered,” this official said. But Trump also did not want America’s Kurdish allies to think, wrongly, that the U.S. would stop a Turkish invasion.

“(We) ... don’t want to give them the impression we’re going to militarily dig in and stop them. We’re not,” said this official.

The State Department official said Erdogan seemed to believe Trump would give the OK for Turkey’s planned incursion or even provide American military backing.

“I think (Erdogan) expected the U.S. would provide military support and fix it with the rest of the world. We’re fixing nothing,” said this official. “They get to own the whole thing.”

This official conceded that the Kurds were worried and disappointed with the U.S. withdrawal. And he struggled to explain how that move squares with the Trump administration’s promise to protect the Kurds from a Turkish attack.

Trump first announced his intention to withdraw U.S. troops from Syria in 2018. Amid a backlash, he later agreed to keep a residual American presence there.

On Monday, Trump did not cite an attack on the Kurds in northern Syria as one of the moves he would consider "off limits." But he warned that Turkey "must, with Europe and others" guard over the captured Islamic State fighters held by the Syrian Democratic Forces if Erdogan sends his forces into Syria.

In helping the U.S. defeat ISIS, the SDF has captured and detained thousands of terrorist fighters in Syria. If they're released, they could return to the battlefield.

The SDF has warned that ISIS sleeper cells are plotting to free about 12,000 militants detained in the region and that they are planning moves against the al-Hol camp, where about 70,000 people are held, including ISIS family members.

The SDF warned that Turkey's planned "invasion" would "have a major negative impact on our war on ISIS and will destroy all the stability achieved during the past years."

Brett McGurk – who served as Trump's envoy to the anti-ISIS coalition before resigning over Trump's 2018 withdrawal announcement – said Turkey "has neither the intent, desire, nor capacity to manage" the massive al-Hol camp, which the inspectors general for the State Department and the Pentagon "warn is the nucleus for a resurgent ISIS."

"Believing otherwise is a reckless gamble with our national security," he said.

Trump's announced withdrawal from Syria in late 2018 also led to the resignation of then-Defense Secretary Jim Mattis.

It's not clear how many American forces remain in Syria at the moment. A spokesman for the Department of Defense, Jonathan Hoffman, said the U.S. would "work with our other NATO allies and coalition partners to reiterate to Turkey the possible destabilizing consequences of potential actions to Turkey, the region, and beyond."

Graham was critical of last year’s plan to pull out of Syria and on Monday he said that if the reports of a withdrawal are accurate it is "a disaster in the making." He warned it will "will be a stain on America's honor for abandoning the Kurds" and "ensures" an "ISIS comeback."

"To say to the American people, 'ISIS has been destroyed in Syria,' is not true," Graham said on Fox News.

He said he would introduce a resolution in the Senate "asking for a reversal of this decision."

Sen. Marco Rubio, R-Fla., said the withdrawal would be a "grave mistake that will have implications far beyond Syria" and would embolden Iran.

In an article for Foreign Affairs magazine, McGurk predicted that a U.S. withdrawal in Syria could allow Iran to establish a "fortified military presence" there. He warned that "if entrenched, it would constitute a major threat to Israel and Jordan, two vital U.S. allies."

He said Tehran's "expansionist ambitions in Syria" were "deterred only by the presence of U.S. troops."

Rep. Eliot Engel, D-N.Y., who chairs the House Committee on Foreign Affairs, called Trump's decision a "gift to Russia, Iran and ISIS, and a staggering betrayal of the Kurdish forces who have been America's good and faithful partners."

The SDF took down fortifications and removed heavy weapons for the border region in a move meant to appease Erdogan as part of an agreement in which American and Turkey would conduct joint patrols along a 78-mile security zone.

"Despite our efforts to avoid any military escalation with Turkey, the flexibility we have shown to move forward with the establishment of the border security mechanism, and the fulfillment of all our obligations in this regard, the US forces did not fulfill their obligations and withdrew their forces from the territories bordering with Turkey," the SDF said in a statement shared by the Kurdish Hawar news agency.

The Kurdish fighters said they "will not hesitate for a moment to defend ourselves" and they called on "Arabs Kurds, Syrians and Assyrians to join forces and stand with their legitimate forces to defend our country against this Turkish aggression."

A spokeswoman for the European Union warned that Turkey's planned military action "will only exacerbate civilian suffering and lead to massive displacement."

Ulrike Demmer, a spokeswoman for German Chancellor Angela Merkel, said Monday that the progress that has been made against the Islamic State "must not be endangered" and warned that Turkey could further destabilize Syria.

Trump said it was time to "bring our soldiers home," adding, "We are 7000 miles away and will crush ISIS again if they come anywhere near us!"

Many critics pointed out that Trump and his family have business interests in Turkey. Some cited a 2012 tweet from Ivanka Trump lauding Erdogan for helping launch a project in Istanbul.

Trump also mentioned the "magnificent" project in a tweet that year.

The unexpected withdrawal comes as Trump faces an impeachment inquiry over allegations that he pressured a foreign government to interfere in the 2020 election.

Contributing: USA TODAY; The Associated Press