The Pentagon says there are roughly 2,000 USA troops in Syria, quadruple the number US officials had acknowledged until now. Those forces left the country after helping local forces retake the Islamic State in Iraq and Syria's (ISIS) one-time de facto capital of Raqqa.
"We are going to maintain our commitment on the ground as long as we need to - to support our partners and prevent the return of terrorist groups", Pentagon spokesman Eric Pahon said.
The US military said it would also continue to support its local allies, the Syrian Democratic Forces, as they move to stabilize eastern Syria following a series of military victories over ISIS.
The actual number of troops has been in decline for months.
U.S. officials are refusing to put precise timelines on the new phase of the anti-IS mission.
"We will be in Syria as long as it takes to make sure that ISIS is not afforded the ability to reestablish safe havens and plan and conduct attacks", he said of the Trump administration's "conditions-based strategy".
However, he promised to be more open in terms of how many troops deployed.
ISIS has lost about 97 percent of the land it once controlled across Syria and Iraq, Manning said Wednesday, but small pockets of fighters remain in both countries.
Pentagon officials in August, for example, acknowledged that there are about 11,000 US troops now serving in Afghanistan - well above the 8,400 forces previously reported.
The United States now has approximately 2,000 troops on the ground in Syria, where they have been helping train and advise Kurdish and Arab partner forces in the fight against IS.
Manning announced the new official figure for American servicemembers in Syria on Wednesday, saying Defense Secretary Jim Mattis wants to increase transparency about where and how troops are deployed.
But U.S. military operations in Syria are expected to continue.
"The troops are changing their stance as a result", he said.
As troops enter a stabilization phase, USA officials said there will not be clear front lines as American troops support efforts to remove mines and other explosives, return displaced people to their homes and support the growth of indigenous institutions in former IS-held territory that hasn't been seized by President Bashar al-Assad - who has long called the U.S. presence illegitimate - or Russian and Iranian forces backing the embattled Syrian leader.
He reiterated USA support for the United Nations -led talks in Geneva peace talks. The department had previously omitted forces on temporary rotation from the count, along with a number of other groups. "Their collective actions call into question their commitment to dealing a lasting defeat to [IS] and other extremist groups".