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American forces killed an Islamic State (IS) group leader and another of the group's members in a strike in Syria, the US military said on Friday.

Washington has stepped up military action against the jihadist group since the fall of Bashar al-Assad's government earlier this month, hitting areas that were shielded by Syrian and Russian air defences before a lightning offensive by rebels who now control the country.

The strike took place Thursday in Deir Ezzor province in eastern Syria, killing IS leader "Abu Yusif" and another operative, the US Central Command (CENTCOM) said on social media, without providing further details on the two jihadists.

"This airstrike is part of CENTCOM's ongoing commitment, along with partners in the region, to disrupt and degrade efforts by terrorists to plan, organize, and conduct attacks," CENTCOM said.

The strike "was conducted in an area formerly controlled by the Syrian regime and Russians," it added.

The United States has for years carried out periodic strikes and raids to help prevent a resurgence of IS but has launched dozens of strikes since Assad's fall.

On 8 December, the day rebels took the capital Damascus, Washington announced strikes on over 75 IS targets that 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."

The announcement of the latest strike came a day after the United States said it had this year doubled the number of troops it has in Syria as part of the anti-IS fight.

The United States had for years said it has some 900 military personnel in the country as part of international efforts against the jihadist group, which seized swathes of territory there and in neighbouring 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.