"The United States will deploy approximately 200 additional US forces to Syria," Carter said, adding that the new contingent would include special operations forces, trainers, advisers, and explosive ordinance disposal teams.
"This latest commitment of additional forces within Syria is another important step in enabling our partners to deal ISIL a lasting defeat," Carter said, using the government's preferred acronym for the terror organization.
Carter made the announcement while speaking at the Manama Dialogue, a regional security summit hosted in the capital of the Gulf state of Bahrain.
The decision will nearly double the number of US troops authorized to operate in Syria. President Barack Obama had in April set a cap of 300 for the number of military advisers permitted in Syria, though the actual number of advisers on the ground is thought to be fewer than that.
Fight against ISIS
Forces in Syria have been primarily working with the Syrian Democratic Forces (SDF), an umbrella organization that includes Kurdish and Arab units.
Those fighters have enjoyed some success against ISIS, coming within 15 miles of Raqqa, according to Carter.
A senior defense official told CNN that the actual adjustment to the troop cap would be 203 personnel and that their purpose was to boost the assault on Raqqa.
The deployment "had been anticipated for some time" the official added, as the final battle looms against the several thousand ISIS fighters believed to be in Raqqa.
The new deployment of American personnel could help in the final assault on that city by enabling the training of additional fighters that could be key to ejecting ISIS.
"We're getting into what is predominantly traditionally more Arab territory," so more Arab fighters are needed, the defense official said.
"We've always been very conscious of the fact that Raqqa is an Arab city."
The official noted that "more and more Arab fighters have wanted to be part of the coalition we have fighting in northeast Syria," but said that this influx has presented challenges in terms of training and equipping these new recruits, thereby requiring the additional US personnel.
Col. John Dorrian, a spokesman for the counter-ISIS coalition, told reporters Thursday that, in anticipation of the push on Raqqa, the SDF was boosting its ranks with more than 1,000 new recruits.
Airstrikes ahead of offensive
The United States is also using its air power to lay the ground for the coming battle. Dorrian said coalition aircraft had conducted nearly 300 strikes against ISIS in support of the offensive, with America's Syrian allies capturing more than 270 square miles of territory.
Carter drew attention to a counter-ISIS strike on Thursday that involved the destruction of 168 oil tanker trucks. Carter called it "the largest airstrike of this kind to date."
But US military officials anticipate a tougher fight in the days ahead.
"Daesh considers Raqqa their capital in Syria, so we expect resistance to stiffen as forces move closer to the city," Dorrian told reporters last month, using another name for the terror group.
And even after Raqqa and Mosul are captured, Carter predicted that the fight against ISIS would continue for some time.
"There will still be much more to do after that to make sure that once defeated, ISIL stays defeated," he said.