WASHINGTON/BEIRUT (REUTERS) - The United States will likely end its air campaign against the Islamic State in Iraq and Syria (ISIS) when it pulls out troops, US officials said on Thursday (Dec 20), as President Donald Trump defended the planned withdrawal against criticism from allies abroad and at home.

Four US officials, who spoke to Reuters on condition of anonymity, said the troop withdrawal is expected to mean an end to the US air campaign against ISIS in Syria. The US-led air war has been vital to crushing the militants there and in neighbouring Iraq, with more than 100,000 bombs and missiles fired at targets in the two countries since 2015.

Still, one US official said a final decision on the air campaign had not been made and did not rule out some kind of support for partners and allies.

The US told the United Nations Security Council it was committed to the "permanent destruction" of ISIS in Syria and would keep pushing for the withdrawal of Iranian-backed forces in the country.

The roughly 2,000 US troops in Syria, many of them special forces, were ostensibly helping to combat ISIS but were also seen as a possible bulwark against Syrian President Bashar al-Assad, who has retaken much of the country from his foes in the civil war, with military help from Iran and Russia.

The Kurdish-led Syrian Democratic Forces (SDF), which have been fighting ISIS with US support for three years, said the withdrawal of troops would let the militants regroup at a critical stage and leave Syrians stuck between "the claws of hostile parties" fighting for territory in the civil war.

The SDF 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 committed to restoring Mr Assad's control over the whole country.

The SDF said the battle against ISIS had reached a decisive phase that required more support, not a precipitate US withdrawal.

THREAT ALIVE

ISIS declared a caliphate in 2014 after seizing parts 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 US estimates, the group oversaw about 100,000 sq km of territory, with about eight million people under its control and estimated revenues of nearly $1 billion a year.

A senior US official last week said the group was down to its last 1 per cent 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.

Nato allies France and Germany said Washington's withdrawal from Syria risks damaging the fight against ISIS, the militant group that had seized swathes of Iraq and Syria but has now been squeezed to a sliver of Syrian territory.

France, a leading member of the US-led coalition against Islamic State, said it would keep its troops in northern Syria for now and contested Mr Trump's assertion that the group has been defeated in the country.

"Islamic State has not been wiped from the map nor have its roots. The last pockets of this terrorist organisation must be defeated militarily once and for all," French Defence Minister Florence Parly said on Twitter.

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.

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

He also questioned what Mr Trump's announcement meant in practical terms, saying there was no sign yet of a withdrawal of US forces, whose presence in Syria Moscow calls illegitimate.

Israel will escalate its fight against Iranian-aligned forces in Syria after the withdrawal of US troops, Prime Minister Benjamin Netanyahu said.

While Turkey has not commented directly on Mr Trump's decision, an end to the US-Kurdish partnership will please Ankara.

Kurdish militants east of the Euphrates river 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 has intervened to sweep YPG and ISIS 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 US forces.