The U.S.-led coalition against Islamic State said on Thursday additional U.S. forces had been deployed to Syria to accelerate the defeat of Islamic State in its Syrian base of operations at Raqqa city.
Confirming media reports, coalition spokesman U.S. Air Force Colonel John Dorrian said the additional forces would be working with local partners in Syria - the Syrian Democratic Forces (SDF) and the Syrian Arab Coalition - and would not have a frontline role.
The SDF includes the Kurdish YPG militia.
The additional forces that had arrived in "the last few days" comprised a Marines artillery unit and Army Rangers.
"We are talking about an additional 400 or so forces in total, and they will be there for a temporary period," Dorrian said by telephone. The deployment was on top of an existing 500 already in Syria, he said.
The SDF is the main U.S. partner in the war against Islamic State in Syria. Since November it has been working with the U.S.-led coalition to encircle Raqqa.
This week, the SDF cut the road between Raqqa and the jihadists' stronghold of Deir al-Zor province - the last main road out of the city.
Dorrian said the effort to isolate Raqqa was "going very very well" and could be completed in a few weeks. "Then the decision to move in can be made," he said.
The additional forces aimed to "provide more artillery and to expedite the defeat of ISIS in Raqqa", he said.
"We have had what I would describe as a pretty relentless air campaign to destroy enemy capabilities and to kill enemy fighters in that area already. That is something that we are going to continue and intensify with this new capability."
Dorrian said the Army Rangers were on a different mission to the Marines in a previously announced deployment near the city of Manbij to "create some reassurance" for U.S.-allied Turkey and U.S. partners in Syria - a reference to the SDF.
Turkey views YPG as a threat to its national security and says that the Kurdish militia maintains a presence in Manbij. The YPG denies this. Fearing deepening Kurdish influence in northern Syria, Turkey has been pressing Washington for a role in the final assault on Raqqa.
Dorrian said a possible role for Turkey "remains a point of discussion at military leadership and diplomatic levels".
"We have always said we are open to a role for Turkey in the liberation of Raqqa and will continue that discussion to whatever logical end there is."