US officials estimate that there are up to 4,000 militants in Raqqa.

The United States has sent a Marine Corps artillery battery into Syria to help defeat ISIS militants in Raqqa, a US official said Wednesday.

The official said troops from the 11th Marine Expeditionary Unit had deployed a battery of 155mm Howitzers to an outpost in Syria. The Marines “are ready to conduct their mission” to support the offensive on Raqqa, the official told AFP, confirming a report in the Washington Post. The move marks a significant step for conventional US forces in Syria.

Currently, the United States has limited its ground troops to about 500 special operations fighters whose job has been to train and assist the SDF, a local Arab-Kurdish alliance fighting ISIS.

According to the Post, the artillery deployment had been in the works “for some time” and was not part of President Donald Trump’s demand for a new plan to increase the pace of the anti-ISIS war. The artillery will allow the Marines to help support the SDF offensive, which has been gradually closing in on Raqqa since last fall.