"Even as the end of the physical caliphate is... now coming into sight, the end of ISIS will be a much more long-term initiative," a U.S. official said.
The United States’ military forces have begun withdrawing from Syria as U.S. officials said the U.S. is considering removing all troops on the ground as it ties up a military campaign to retake territory previously held by the Islamic State group.
"We have started returning United States troops home as we transition to the next phase of this campaign," White House spokeswoman Sarah Sanders said, suggesting that the country would remain engaged to some degree.
Her remarks came shortly after U.S. President Donald Trump tweeted, "We have defeated ISIS in Syria, my only reason for being there," using an acronym for the Islamic State group.
It was not clear from Sanders' statement whether all of the roughly 2,000 U.S. troops in Syria would leave or by what deadline. A U.S. official told Reuters that the U.S. State Department is evacuating all of its personnel from Syria within 24 hours.
"The United States and our allies stand ready to re-engage at all levels to defend American interests whenever necessary, and we will continue to work together to deny radical Islamist terrorist territory, funding, support," Sanders said.
A decision to pull out completely, if confirmed, would upend assumptions about a longer-term U.S. military presence in Syria, which senior U.S. officials have advocated for to ensure that the Islamic State group does not reemerge.
The move could also undercut U.S. leverage in the region and undermine diplomatic efforts to end a civil war in Syria that has killed hundreds of thousands of people and displaced around half of the country's population, which stood at 22,000,000 people before the war began in 2011.
The Pentagon declined to comment, saying only that it continued to work with partners in the region.
U.S. Republican Senator Lindsey Graham, who often sides with Trump, said a withdrawal would have "devastating consequences" for the United States in the region and throughout the world.
"An American withdrawal at this time would be a big win for ISIS, Iran, (President) Bashar al Assad of Syria, and Russia," Graham said.
The Syrian government has always rejected the U.S. intervention into the country saying that it had not asked for Washington to send troops and in fact the U.S. and its allies in the region were, in fact, aiding extremist anti-government groups in their quest to oust the government of Bashar Assad.
Many of the remaining U.S. troops in Syria are special operations forces who have been working closely with Kurdish and Arab militias who are in an alliance known as the Syrian Democratic Forces, or SDF.
Over the past several years, the SDF partnership has brought the defeat of Islamic State in Syria, but has also outraged Turkey, a NATO ally which views Kurdish YPG forces in the alliance as an extension of a militant group fighting inside Turkey.
A complete withdrawal of U.S. troops from Syria would still leave a sizeable U.S. military presence in the region, including about 5,200 troops across the border in Iraq. Much of the U.S. campaign in Syria has been waged by warplanes flying out of Qatar and other locations in the Middle East.
Under the administration of Barack Obama, the United States formed a coalition of its allies in the region to fight the Islamic State group in both Syria and Iraq, mainly through airstrikes. The campaign has been widely criticized for a mounting death toll of civilians over the years since September 2015. According to human rights groups monitoring the situation in Syria, over 5,000 civilians have been killied by the coalition's airstrikes.