"The campaign to defeat ISIS is now in a new phase in Iraq and Syria", Col. Robert Manning, a Pentagon spokesman, said, using an acronym for the Islamic State. US forces in Syria and Iraq are "trending downward", he added, without specifying whether the 2,000 troops in Syria included the 400 Marine artillerymen that the Pentagon announced last week would soon leave the country.
The 1,500 or so previously undisclosed troops in Syria include traditional combat forces like infantry, artillery and forward air controllers trained to call in airstrikes, as well as support personnel, said Eric Pahon, a Pentagon spokesman.
The US-led anti-ISIS military coalition announced Monday that coalition warplanes had conducted 28 airstrikes against ISIS targets near Abu Kamal, Syria, since Friday, striking ISIS tactical units, vehicles and heavy weapons.
"We're changing the composition of our forces to something that supports the diplomats and the Geneva process", Mattis told reporters while en route to meetings in Egypt.
The US military has increased the number of troops in Syria from 500 to 2,000, the Pentagon says, adding that the "conditions-based" military presence is justified by the need to "stabilize" liberated areas.
"ISIS left a minefield when they started walking out", he said.
U.S. troops are known to be training and providing support to the Arab-Kurdish fighters of the Syrian Democratic Forces, who have been battling the Islamic State group in northern and eastern Syria.
Furthermore, Russia and Syria do not "appear to be serious about the withdrawal of Iranian-backed militias" operating in Syria, Pahon said.