The official said troops from the 11th Marine Expeditionary Unit had deployed a battery of 155mm Howitzers to an outpost in Syria.
The move marks a significant step for conventional US forces in Syria.
The artillery will allow the Marines to help support the SDF offensive, which has been gradually closing in on Raqa since last fall.
The Pentagon this week announced that it had also sent extra troops into Manbij to prevent competing factions fighting each other in the former IS stronghold.
US officials estimate there to be up to 4,000 jihadists in Raqa, a city of about 300,000 residents.