WASHINGTON/BEIRUT - The United States has sent a Marine Corps artillery battery into Syria to help defeat the Islamic State group in Raqa, the capital of their supposed caliphate, a US official has said.

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 Raqa, 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 IS.

The toll in air strikes thought to have been conducted by a US-led coalition on a militant-held northern Syrian village on Thursday has risen to 23 civilians killed, a monitor said.

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“The raids hit the village of Al-Matab after midnight and were likely carried out by the coalition,” said Rami Abdel Rahman of the Syrian Observatory for Human Rights. Coalition spokesman US Air Force Colonel John Dorrian said the additional US forces would be working with local partners in Syria - the Syrian Democratic Forces (SDF) and the Syrian Arab Coalition - and would not have a front line role. The additional deployment comprises a total of 400 US forces - both Marines and Army Rangers. It adds to around 500 US military personnel already in Syria, Dorrian said.

The SDF, which includes the Kurdish YPG militia, is the main US partner in the war against Islamic State insurgents in Syria. Since November it has been working with the US-led coalition to encircle Raqqa, IS’s main urban bastion in Syria.

This week, the SDF cut the road between Raqqa and the militants’ stronghold of Deir al-Zor province - the last main road out of the city. Islamic State is also being fought in Syria by the Russian-backed Syrian military, and by Syrian rebel groups fighting under the Free Syrian Army banner with Turkish backing in northern Syria and Jordanian backing in southern Syria.

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 had arrived in “the last few days”, he told Reuters by telephone.

The artillery will help “expedite the defeat of ISIS in Raqqa”, he said, using another acronym for Islamic State. The Marines were armed with 155-millimetre artillery guns. Asked if they had been used yet, Dorrian said he did not believe so.

“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.”

“We are talking about an additional 400 or so forces in total, and they will be there for a temporary period,” he said.

A Kurdish military source told Reuters the extra US forces were deployed as part of a joint plan between the SDF and US-led coalition to capture Raqqa, and further US reinforcements were expected to arrive in the coming few days.

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 US-allied Turkey and US partners in Syria - a reference to the SDF.

Turkey views the YPG as a threat to its national security and says 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.”