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The United States doubled the number of troops it has in Syria to around 2,000 earlier this year, the Pentagon said Thursday.

Washington has for years said it has some 900 military personnel in Syria as part of international efforts against Islamist groups, 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.

'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.

Washington, which also says it has some 2,500 troops in Iraq, has for years carried out periodic strikes and raids justifying the move as a prevention against the 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 defences before a lightning offensive by rebels who now control the country.

When rebels toppled Bashar al-Assad on December 8, 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'.