WASHINGTON (AP) — The Trump administration will soon withdraw all of the approximately 2,000 American troops from Syria, a U.S. official said Wednesday as President Donald Trump declared victory in the mission to defeat Islamic State militants there.

Planning for the pullout has begun and troops will begin leaving as soon as possible, said the official, who was not authorized to publicly discuss military planning and spoke on condition of anonymity.

On Wednesday, as Vice President Mike Pence met with top military leaders in the Pentagon, Trump tweeted: “We have defeated ISIS in Syria, my only reason for being there during the Trump Presidency.”

That declaration of victory is far from unanimous, and the withdrawal decision immediately triggered demands from Congress — including Republicans — for more information and a formal briefing on the matter. Sen. Lindsay Graham, R-S.C., who just returned from Afghanistan, said he was meeting with Defense Secretary Jim Mattis late in the day.

The decision will fulfill Trump’s long-stated goal of bringing troops home from Syria, but military leaders have pushed back, arguing that the IS group remains a threat and could regroup as it battles in Syria’s long-running civil war.

Trump has argued for the withdrawal since he was a presidential candidate. But the decision underscores the division between him and his military advisers, who have said in recent weeks that pockets of IS militants remain and U.S. policy has been to keep troops in place until the extremists are eradicated.

Israeli Prime Minister Benjamin Netanyahu, who remains concerned about Iranian efforts in the area, reacted in noncommittal fashion after talking with Trump by telephone.

“This is, of course, an American decision,” he said. Israel will learn of the timetable and manner of withdrawal, he said, and no matter what “we will safeguard the security of Israel and protect ourselves from this arena.”

Leading Republican senators reacted with displeasure to the news.

Graham, typically a Trump backer, said he was “blindsided” by the report and called the decision “a disaster in the making.” He said, “The biggest winners in this are ISIS and Iran.”

Republican Sen. Marco Rubio of Florida said the withdrawal would be a “grave error with broader implications” beyond the fight against IS.

Just last week, the U.S. special envoy to the anti-ISIS coalition, Brett McGurk, said U.S. troops would remain in Syria even after the Islamic State was driven from its strongholds.

“I think it’s fair to say Americans will remain on the ground after the physical defeat of the caliphate, until we have the pieces in place to ensure that that defeat is enduring,” McGurk told reporters on Dec. 11. “Nobody is declaring a mission accomplished. Defeating a physical caliphate is one phase of a much longer-term campaign.”

And two weeks ago Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, said the U.S. still has a long way to go in training local Syrian forces to prevent a resurgence of IS and stabilize the country. He said it will take 35,000 to 40,000 local troops in northeastern Syria to maintain security over the long term, but only about 20 percent of them have been trained.

Trump’s national security adviser, John Bolton, said in September that the U.S. would keep a military presence in Syria as long as Iran is active there. “We’re not going to leave as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias,” he said.

James Stavridis, a former Navy admiral who served as top NATO commander, tweeted Wednesday that “Pulling troops out of Syria in an ongoing fight is a big mistake. Like walking away from a forest fire that is still smoldering underfoot. Big winner is Iran, then Russia, than Assad. Wrong move.”

The withdrawal decision, however, is likely to be viewed positively by U.S. ally Turkey, and comes following several conversations between Trump and Turkish President Recep Tayyip Erdogan over the past several weeks. The two spoke at the G-20 summit in Argentina and in a phone call last Friday. The Turks have targeted U.S.-backed Kurdish troops along the Syria-Turkey border, which Turkey considers an insurgent threat. A U.S. withdrawal — including the end of joint U.S. and Turkish patrols along the border — could open the door for more Turkish operations against the Syrian rebels.

Erdogan said Monday he had gotten “positive answers” from Trump on the situation in northeast Syria where he has been threatening a new operation against American-backed Syrian Kurdish fighters.

Just hours before the withdrawal decision became public, the State Department announced late Tuesday that it had approved the sale of a $3.5 billion Patriot missile defense system to Turkey. The Turks had complained that the U.S. was slow walking requests for air defenses and had signed a deal with Russia to buy a sophisticated system in a deal that Washington and Ankara’s other NATO partners strongly opposed.

Completion of the deal with Russia for the S-400 system would have opened up Turkey to possible U.S. sanctions and driven a major wedge between the allies. It was not immediately clear if there was a connection between the Patriot sale and the decision on U.S. troops.

Although the withdrawal decision doesn’t signal an end to the American-led coalition’s fight against the Islamic State, it also will likely erode U.S. leadership of that 31-nation effort. The administration had been preparing to host a meeting of coalition foreign ministers at the State Department early next year.

The decision to withdraw was first reported by The Wall Street Journal.

Jennifer Cafarella, a Syria expert at the Institute for the Study of War, said the withdrawal is likely to create new problems.

“The bottom line is that the American withdrawal from eastern Syria will create a power vacuum that will lead to a new phase of international conflict in Syria,” she said in a telephone interview. She predicted that the Russians, the Iranians, Syrian President Bashar Assad and the Turks will compete for the terrain and resources previously under U.S. control “at the expense of” the Syrian Kurds who have partnered with U.S. forces against IS.

The U.S. first launched airstrikes against IS fighters in Syria in 2014. In the years that followed, the U.S. began partnering with Syrian ground forces to fight the extremists.

The Pentagon recently said that IS now controls just 1 percent of the territory it originally held.

Hikmat Habib, a Syrian Kurdish, said he has not yet seen any evidence of a withdrawal, but added that the move is “part of the disagreement between influential states in Syria: American-Turkish disagreements, American-Russian disagreements.” He said the U.S. also wants to end the Iranian presence in Syria and prevent it from spreading.

“We may see a new phase in the Syrian crisis in the coming days, and its features will be shown as Daesh is now limited to specific areas,” he said, using another name for IS. “There may be a new mechanism for resolving the Syrian crisis” based on political processes and U.N. resolutions.