The Pentagon says there are about 2,000 US troops in Syria.
"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. Manning declined to say how many American troops were in Syria or Iraq at the height of urban combat operations - those in Raqqa or Mosul, for example - that required larger footprints of USA combat advisers.
Manning noted that troop counts in both countries are "trending downward" as the conflict against ISIS takes on a "new phase", with the extremist group holding less than three percent of the territory it once controlled in both countries.
USA officials are refusing to put precise timelines on the new phase of the anti-IS mission.
A Pentagon official said on Tuesday the USA military plans on staying in Syria as long as necessary to ensure the Islamic State group does not return.
However, he promised to be more open in terms of how many troops deployed. Last week, about 400 Marines in an artillery unit - 1st Battalion, 10th Marines - that was carrying out strikes against the Islamic State in the city of Raqqa returned to the United States.
The Defense Department did not immediately clarify how those forces were accounted for or how United States troops would be positioned throughout the country during the stabilization effort, which will proceed as the Trump administration backs UN-led talks to decide the political future of Syria. As part of that effort, the Pentagon announced in August it had about 3,000 more troops in Afghanistan than acknowledged under then-President Barack Obama.
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.
The Defense Department under Secretary Jim Mattis had promised to provide more accurate estimates of US troops stationed overseas. It's not clear if that release and subsequent reports by Al-Monitor and other news outlets prompted the long-planned revision. Critics say the policy masked the true extent of USA forces fighting IS and prevented the United States military from using support troops, such as mechanics and equipment maintainers in-theater, creating greater reliance on contractors.
But with Syrian negotiators walking out on the UN-backed Geneva talks this week, the Pentagon offered its heaviest criticism of Russian-led military efforts in recent memory. The department had previously omitted forces on temporary rotation from the count, along with a number of other groups.
Unlike Russia, the United States is not in Syria at the request or approval of the Damascus government.