The extra troops will include special operations forces and are in addition to 300 US troops already authorized for the effort to recruit, organise, train and advise local Syrian forces to combat IS.
Addressing a security conference in Bahrain, Carter said the extra troops will help the 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.
"These uniquely skilled operators will join the 300 US special operations forces already in Syria, to continue organizing, training, equipping, and otherwise enabling capable, motivated, local forces to take the fight to ISIL," Carter said in his address to the IISS Manama Dialogues in the Bahraini 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.