By Phil Stewart and Ellen Francis

WASHINGTON/BEIRUT (Reuters) – The United States will end its air campaign against Islamic State in Syria when it pulls out troops, U.S. officials said, sealing an abrupt reversal of policy which has alarmed Western allies as well as Washington’s Kurdish battle partners.

The Syrian Democratic Forces (SDF), which have been fighting Islamic State with U.S. support for three years, said President Donald Trump’s withdrawal of troops would grant the militants breathing space to regroup at a critical stage in the conflict and leave Syrians stuck between “the claws of hostile parties” fighting for territory in the seven-year-old war.

Trump’s announcement on Wednesday upended a central pillar of American policy in the Middle East and stunned U.S. lawmakers and allies.

Western allies including France, Britain and Germany described Trump’s assertion of victory as premature. France, a leading member of the U.S.-led coalition, said it would keep its troops in northern Syria for now because Islamic State militants had not been wiped out.

Trump defended his decision on Thursday, tweeting that he was fulfilling a promise from his 2016 presidential campaign to leave Syria. The United States was doing the work of other countries, including Russia and Iran, with little in return and it was “time for others to finally fight,” he wrote.

U.S. officials said Trump’s order to withdraw troops also signifies an end to the U.S. air campaign against Islamic State in Syria, which has been critical to rolling back the militants there and in neighboring Iraq, with more than 100,000 bombs and missiles fired at targets in the two countries since 2015.

The SDF, supported by about 2,000 U.S. troops, are in the final stages of a campaign to recapture areas seized by the militants.

But they face the threat of a military incursion by Turkey, which considers the Kurdish YPG fighters who spearhead the force to be a terrorist group, and Syrian forces – backed by Russia and Iran – committed to restoring President Bashar al-Assad’s control over the whole country.

The SDF said the battle against Islamic State had reached a decisive phase that required more support, not a precipitate U.S. withdrawal.

THREAT ALIVE

France’s Europe Minister Nathalie Loiseau said: “For now, of course we are staying in Syria because the fight against Islamic State is essential.”

France has about 1,100 troops in Iraq and Syria providing logistics, training and heavy artillery support as well as fighter jets. In Syria, it has dozens of special forces, military advisers and some foreign office personnel.

A British junior defense minister said he disagreed with Trump. “(Islamic State) has morphed into other forms of extremism and the threat is very much alive,” Tobias Ellwood said.

Islamic State declared a caliphate in 2014 after seizing large swathes of Syria and Iraq. The hardline group established its de facto capital in the Syrian city of Raqqa, using it as a base to plot attacks in Europe.

According to U.S. estimates, the group oversaw about 100,000 square kms (39,000 square miles) of territory, with about 8 million people under Islamic State control. It had estimated revenues of nearly $1 billion a year.

A senior U.S. official last week said the group was down to its last 1 percent of the territory it once held. It has no remaining territory in Iraq, although militants have resumed insurgent attacks since the group’s defeat there last year.

Russian President Vladimir Putin said he largely agreed with Trump that Islamic State had been defeated in Syria but added there was a risk it could recover.

He also questioned what Trump’s announcement would mean in practical terms, saying there was no sign yet of a withdrawal of U.S. forces whose presence in Syria Moscow says is illegitimate.

Israel will continue to act “very aggressively against Iran’s efforts to entrench in Syria,” Israeli Prime Minister Benjamin Netanyahu said.

Neighboring Turkey, which has threatened an imminent military incursion targeting the U.S.-allied Kurdish YPG fighters in northern Syria, has not commented directly on Trump’s decision, although an end to the U.S.-Kurdish partnership will be welcomed in Ankara.

Kurdish militants east of the Euphrates in Syria “will be buried in their ditches when the time comes”, state-owned Anadolu news agency reported Defence Minister Hulusi Akar as saying. Turkey considers the YPG a terrorist group and an extension of the outlawed Kurdistan Workers Party (PKK).

Turkey has intervened to sweep YPG and Islamic State fighters from parts of northern Syria that lie west of the Euphrates over the past two years. It has not gone east of the river, partly to avoid direct confrontation with U.S. forces.

(Additional reporting by Daren Butler in Istanbul and John Irish in Paris; Writing by Dominic Evans; Editing by Janet Lawrence)