WASHINGTON (Reuters) - U.S. President Donald Trump’s order to withdraw U.S. troops from Syria also is expected to signify an end to the U.S. air campaign against Islamic State there, U.S. officials told Reuters on Thursday, speaking on condition of anonymity.

An F/A-18 fighter jet lands on the USS Harry S. Truman aircraft carrier in the eastern Mediterranean Sea, May 4, 2018. REUTERS/Alkis Konstantinidis

One U.S. official, however, cautioned that a final decision had not yet been made, and did not rule out some kind of support for partners and allies. France, for example, has said it will continue to fight in Syria.

Trump’s surprise decision on Wednesday to completely withdraw U.S. troops from the country triggered criticism from some Republicans and concern from America’s allies.

An end to the U.S. air war will likely heighten fears that Islamic State, which has lost almost all of the territory it once controlled, could be given space to regroup.

The announcement of a withdrawal of troops on the ground had not necessarily meant an end to the air war since, unlike ground troops, major U.S. air assets are not based in Syria and instead fly into the country from nearby nations. The U.S. air operations center for the air war is located in Qatar.

The U.S.-led air war has been critical to rolling back Islamic State and keeping pressure on the militant group in Iraq and Syria, with more than 100,000 bombs and missiles fired at targets in the two countries since 2015, according to Air Force data.

U.S. officials told Reuters that the timing of the end of the air campaign would be linked to the withdrawal of the U.S. forces but declined to set a date for when that would happen.

It was also not clear whether there was any wiggle room to alter the decision, since Trump has not spoken in public on it directly. The U.S. military is inclined, whenever possible, to support allies battling a common enemy.

The Pentagon declined to speculate.

“As long as there are U.S. troops on the ground we will conduct air and artillery strikes in support of our forces. We will not speculate on future operations,” U.S. Central Command said in a statement.

U.S. air assets are essential not only for offensive strikes against militants but also to defend U.S. troops on the ground. That role, known as “force protection,” will be paramount to ensuring an orderly and safe exit by American forces from Syria.

As of August 2017, the U.S.-led coalition had carried out more than 11,200 strikes in Syria, according to the U.S. military. Since then, hundreds more have been carried out.

UNANSWERED QUESTIONS

Trump’s decision has left many questions unanswered, including how U.S. allies and partners will fill the void.

Some of Trump’s fellow Republicans strongly criticized the move, saying they were not briefed ahead of time and that the move strengthened the hand of Russia and Iran, which both support Syrian President Bashar al Assad.

The U.S.-backed Syrian Democratic Forces (SDF) said President Donald Trump’s withdrawal of all U.S. troops would also leave Syrians stuck between “the claws of hostile parties” fighting for territory in the seven-year-old war there.

U.S. officials have told Reuters that U.S. commanders on the ground are also concerned about the impact of a quick withdrawal and were surprised by the decision.

Trump’s move also drew criticism from some U.S. allies, including Britain and France, which said Islamic State militants had not been defeated and that its troops would remain in Syria.

Trump defended his decision to declare victory over Islamic State militants in Syria and completely withdraw. In early morning tweets, Trump said he was fulfilling a promise from his 2016 presidential campaign to leave Syria.