BEIRUT (AP) -- As U.S.-backed forces close in on the Islamic State group in the northern Syrian town of Manbij, the extremists have used hundreds of residents as human shields, laying mines and shooting those trying to flee the town near the Turkish border.

Manbij, with a pre-war population of 75,000 to 100,000 people, is a major transit hub between the border and the IS group's de facto capital, Raqqa. As the fighting has dragged on for more than two months, it has emerged as a major test for the U.S.-backed and Kurdish-led militia known as the Syrian Democratic Forces.

"Manbij will inform us as to how we are going to fight in Raqqa, as Ramadi has informed us how we'll fight in Mosul," Lt. Gen. Sean MacFarland, the top U.S. commander for the fight against IS, told reporters late Wednesday.

On Thursday, SDF forces stormed one of the streets in which IS militants have shielded themselves with residents, freeing hundreds after brief clashes and leaving the militants holed up in an ever-smaller corner of the northern district of Sarab.

"They broke through the line of defense of the Daesh mercenaries" and freed 400 civilians, said Ekram Berkat, a reporter in Manbij with the Kurdish Hawar news agency, referring to IS by an Arabic acronym. "Daesh are only left in a couple of streets in Sarab," where they have been holed up for the last week.

Berkat said around 1,000 more civilians are being held by IS, which has refused repeated offers of safe passage out of the town in exchange for their release.

The Kurdish-led SDF, which includes hundreds of Arab fighters, has swept across northern Syria in recent months, driving IS from a number of strongholds near the Turkish border. The U.S. is training its fighters and U.S. special forces are on the ground advising them. The U.S.-led coalition has carried out numerous airstrikes in support of the operation.

MacFarland, praised the SDF as a "respectable" fighting force in a difficult battle, which he said has claimed the lives of some 2,000 IS fighters, including many foreigners. He said the SDF has also suffered casualties, without elaborating.

Chris Kozak, a researcher with the Washington-based Institute for the Study of War, said the fighting in Manbij has dragged on not only because of the use of human shields, but because of the dense urban terrain.

"Manbij gives an indication of how messy and drawn out it actually will be to use the Syrian Democratic Forces as an instrument to try to seize Raqqa city," Kozak said. Raqqa is a majority Arab city twice the size of Manbij, with a large population of internally displaced people who moved there after IS took control.

The SDF in Manbij has relied on U.S.-led airstrikes, of which there were 15 on Wednesday alone. On July 19, as many as 100 civilians were killed in U.S. airstrikes on Manbij and a nearby village. A U.S. investigation is underway.

The protracted fighting has meanwhile devastated much of the town.

Mustafa Bali, a Syrian Kurdish activist who also traveled with the SDF, said IS torched the town's ancient covered market, destroying and vandalizing shops. Berkat, the reporter, said graffiti around the town left by IS proclaims: "This is what the people of Manbij deserve for not fighting on our side." SDF spokesman Sharfan Darwish said the militants also burned the civil registry records in government offices.

Kozak said that although the SDF forces in Syria consisted of an estimated 2,500 Arabs and just 500 Kurds, they still relied on the Kurds for command and logistics.

That could prove problematic for neighboring Turkey, which views the Kurdish fighters as an extension of the Kurdistan Workers' Party, or PKK, a group that is waging a revived insurgency in southeastern Turkey.

In part to assuage Ankara's concerns, the SDF created a Military Council of Manbij to lead the battle, with members chosen to reflect the purported local and Arab nature of the effort.

The Kurds are widely believed to be seeking the further extension of their largely autonomous region in northeastern Syria. But it's unclear whether they would be willing or able to lead the way in a costly assault on Raqqa.

"If you are realistically going to have a force to seize Raqqa, there needs to be much more investment in developing the independent capability of the Syrian Arab component so that it is not seen as a vessel for the Syrian Kurds," Kozak said.