Washington has for years said it has some 900 military personnel in Syria as part of international efforts against the jihadist group, which seized swathes of territory there and in neighboring Iraq before being defeated by local forces backed by a US-led air campaign.

But there are now "approximately 2,000 US troops in Syria" and have been for at least a few months, Pentagon spokesman Major General Pat Ryder told journalists, saying he had just received the updated figure.

"The additional numbers... are considered temporary forces that are there to support the D-ISIS mission, to support the forces that are deployed there longer-term," Ryder said, referring to the mission to defeat the IS group.

Washington -- which also says it has some 2,500 troops in Iraq -- has for years carried out periodic strikes and raids to help prevent a resurgence of the Islamic State group.

But it has stepped up strikes since the fall of Bashar al-Assad's government earlier this month, hitting areas previously shielded by Syrian and Russian air defenses before a lightning offensive by rebels who now control the country.

On December 8 -- the day the rebels took the capital Damascus -- Washington announced strikes on more than 75 IS targets that the US Central Command (CENTCOM) said were aimed at ensuring it "does not seek to take advantage of the current situation to reconstitute in central Syria."

And on Monday, CENTCOM said US forces killed 12 militants from the group in strikes it said were carried out "in former regime and Russian-controlled areas."