As many as 200 more US troops are being sent to Syria to help Kurdish and Arab fighters capture Islamic State's key stronghold of Raqqa, defence secretary Ash Carter said.
The extra troops will include special operations forces and are in addition to 300 already authorised for the effort to recruit, organise, train and advise local Syrian forces to combat IS.
Addressing a security conference in Bahrain, Mr Carter said the extra troops will help local forces in their anticipated push to retake Raqqa, the de facto capital of the extremist group's self-styled caliphate, and to deny sanctuary to IS after Raqqa is captured.
He said President Barack Obama approved the troop additions last week.
"These uniquely skilled operators will join the 300 US special operations forces already in Syria, to continue organising, training, equipping and otherwise enabling capable, motivated local forces to take the fight to Isil," Mr Carter said in his address to the IISS Manama Dialogues in the Bahrain capital Manama.
"By combining our capabilities with those of our local partners, we've been squeezing Isil by applying simultaneous pressure from all sides and across domains, through a series of deliberate actions to continue to build momentum," he said.
The military push in Syria is complicated by the predominant role played by local Kurdish fighters, who are the most effective US partner against IS in Syria but are viewed by Turkey -- a key US ally -- as a terrorist threat.
A senior defence official said the troop boost will give the US extra capability to train Arab volunteers who are joining the Raqqa push but are not well trained or equipped.