US Marines arrived in northern Syria with artillery to support Washington-backed local forces fighting the Islamic State (IS) terror group, according to top officials.

The US-backed fighters are preparing to move in the coming weeks to assault the city of Raqqa, IS' self-declared capital, the officials told media on Wednesday. 

However, the Pentagon and the Marine Corps have declined to confirm the deployment due to security concerns in the region. They have also declined to specify the exact location of the forces or how many are there.

The deployment of the Marines was revealed in a Post report on Wednesday.

"The deployment marks a new escalation in the US war in Syria, and puts more conventional American troops in the battle," the report said.

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-millimeter shells from M777 Howitzers, the officials said on the condition of anonymity.

A similar deployment last year near Mosul, Iraq involved several hundred Marines equipped with artillery guns that fire shells to provide covering fire for advancing forces, media reported.

This is the second major expansion of US ground forces in northern Syria with over 100 Army Rangers were deployed in and around Manbij, Syria.

IS could have as many as 4,000 fighters in Raqqa.