WASHINGTON — The United States is sending an additional 400 troops to Syria to help prepare for the looming fight for Raqqa, the capital of the Islamic State’s self-proclaimed caliphate, American officials said on Thursday.

The increase, which includes a team of Army Rangers and a Marine artillery unit that have already arrived in Syria, represents a near-doubling of the number of American troops there.

The United States military has declined to say how many troops it has deployed in Syria. The formal troop cap is 503, but commanders have the authority to temporarily exceed that limit.

The Rangers’ presence became apparent last weekend when they were seen driving around the northern Syrian town of Manbij in Stryker vehicles and armored Humvees. The Washington Post earlier reported the deployment of the Marine artillery battery.