The move comes as American-backed forces intensify their assault on Raqqa, the Islamic State's capital city.

The Washington Post first reported Wednesday that Marines attached to the MEU -- which is deployed around the Middle East, conducting training in countries including Oman and Djibouti -- had departed their ships and moved into Syria.

A defense official would not speak to the size of the detachment deployed to Syria, but said it included elements of multiple artillery batteries, as well as support personnel, including infantry Marines. It's the first time American artillery support capability has been on the ground in Syria since the fight against the Islamic State began in 2014.