BEIRUT (Reuters) - U.S.-backed Syrian militias said on Thursday they have enough forces to capture the city of Raqqa from Islamic State with support from the U.S.-led coalition, underlining their opposition to any Turkish role in the attack.

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Raqqa is Islamic State’s main base of operations in Syria and the U.S.-backed campaign to capture it has been boosted with the arrival of a Marines artillery unit.

The Syrian Democratic Forces (SDF), which includes the Kurdish YPG militia and Arab groups, have in recent days cut the road from the Raqqa to Islamic State’s stronghold in Deir al-Zor province - the last main road out of the city.

Deeply worried by the YPG’s influence, Ankara is pressing Washington to take part in the final assault on Raqqa.

The SDF says it ruled out any Turkish role during meetings with U.S. officials last month, though Turkey said on Thursday no decision had been made yet and the U.S.-led coalition said a possible Turkish role remained a point of discussion.

“The number of our forces is now increasing, particularly from among the people of the area, and we have enough strength to liberate Raqqa with support from the coalition forces,” Jihan Sheikh Ahmed, an SDF spokeswoman, said.

Turkey views the YPG as an extension of the Kurdistan Workers Party (PKK), which has fought a three-decade insurgency against the Turkish government.

An SDF spokesman told Reuters on Thursday he expected the forces to reach the outskirts of the city within a few weeks. Its forces began the operation to encircle Raqqa in November.

“We have information that the enemy is moving part of its leadership outside the city, as it is also digging tunnels under the ground. We expect they will fortify the city and the terrorist group will depend on street warfare,” Ahmed said.

The SDF and YPG have been the main partners for Washington in its campaign against Islamic State in Syria. The U.S.-led coalition has been providing air support and deployed special forces in Syria to help in the campaign against Islamic State.