Marines from an amphibious task force have left their ships in the Middle East and deployed to Syria, establishing an outpost from which they can fire artillery guns in support of the fight to take back the city of Raqqa from the Islamic State, defence officials said.

The deployment marks another escalation in the US war in Syria, and puts conventional US troops in the battle there. Several hundred Special Operations troops have advised local forces there for months, but the Pentagon has shied away from using conventional forces in Syria.

The force is part of the 11th Marine Expeditionary Unit, which left San Diego on Navy ships in October. The Marines on the ground include part of an artillery battery that can fire powerful 155-millimetre shells from M777 Howitzers, two officials said, speaking on the condition of anonymity because of the sensitivity of the deployment.

The expeditionary unit's ground force, Battalion Landing Team 1st Battalion, 4th Marines, will man the guns and deliver fire support for US-backed local forces who are preparing an assault on the city. Additional infantrymen from the unit are likely to provide security.