KOBANI, Syria (AP) — Advances against the Islamic State group in its stronghold of Raqqa have slowed down amid stiff resistance from the militants, the spokesman for the U.S.-led coalition fighting the group said.

U.S. Army Col. Ryan Dillon estimates there are around 2,000 IS militants in the northern city, saying they are using civilians and children as human shields.

"We know this is not going to be an easy fight," Dillon told The Associated Press in a phone interview Thursday night.

The U.S. has partnered with the Kurdish-dominated coalition fighting force known as the Syrian Democratic Forces in its fight against IS in Syria. In the month since they launched their offensive for Raqqa, they have encircled the militants and breached the heavily fortified Old City, gaining a foothold inside.

Dillon said the SDF is still making steady advances, but acknowledged a slower pace than the first two weeks of the operation, which saw quick and immediate progress. He said the distance between the closest SDF forces on the eastern and on the western fronts is now just under 2 kilometers (1.2 miles).

Dillon said IS is using many of the same tactics it employed in the Iraqi city of Mosul, including the use of tunnel networks, vehicle-borne IEDs, drones and the use of civilians, sometimes children, to prevent coalition forces from striking specific areas in the city.

"We know that it is not going to be an overnight success but the coalition and the SDF will continue to push forward and will be victorious," he said.

Dillon also said the coalition is concerned about Turkish shelling and threats to launch a cross-border operation into Afrin, a Kurdish-controlled enclave near the border with Turkey, saying the SDF should remain focused on defeating IS in Raqqa.

He said, however, that the Raqqa campaign has so far not been affected.

"As far as we know the same amount of forces that were dedicated to defeating IS in Raqqa from the beginning has sustained and has stayed the same," he said.