FILE - In this Friday, July 28, 2017 file photo, a Kurdish man mourns as he sits next to the grave of his friend who was killed while fighting against Islamic State militants in Raqqa, at a cemetery in Kobani, Syria. While Iraq’s Kurds have sparked confrontation with their drive for independence, Kurds in Syria are making major advances toward their more modest goal, entrenching their self-rule.(AP Photo/Hussein Malla, File)

BEIRUT - The U.S.-led coalition battling the Islamic State group said Wednesday that it won't accept a negotiated withdrawal for hundreds of IS militants holed up in the Syrian city of Raqqa, once the extremists' de facto capital.

The remarks by coalition spokesman, Col. Ryan Dillon, came as coalition allies were working out ways to safely evacuate an estimated 4,000 civilians who remain trapped in the city.

The coalition has said IS militants are holding some civilians as human shields, preventing them from escaping as the fight enters its final stages. The city, on the banks of the Euphrates River, has been badly damaged by the fighting, and activists have reported that over 1,000 civilians have been killed there since June.

The United Nations estimates 8,000 people are trapped in Raqqa, and said September was the worst month in 2017 for civilians in Syria.

Dillon said the Raqqa Civil Council, a local administration of Arab and Kurdish officials, was leading the discussions to ensure the safe evacuation of civilians. However, it was not clear with whom the council is speaking inside Raqqa. A Kurdish-led force, the Syrian Democratic Forces, is leading the battle on the ground.

"We are seeing some good progress of civilians that are being able to safely exit Raqqa. The trend has turned into ... a broader effort by the Raqqa Civil Council to get the remaining civilians out of there," Dillon told The Associated Press. He said at least 700 civilians have been evacuated from the city since Monday.

But Dillon added that discussions about the fate of the militants remaining in the city have focused on "unconditional surrender."

A negotiated withdrawal "is absolutely something that we as a coalition would not be a part of or agree with," Dillon added. Between 300 and 400 militants are believed to be holed up in about 4 square kilometres (1.5 square miles) of Raqqa, including in the city's stadium and a hospital, he said.

The stadium is believed to be used by the militants as weapons warehouse and a prison while the hospital is one of their major headquarters.

Dillon said that in the last three weeks, up to 15 militants, including a senior leader, have surrendered in Raqqa, a trend also seen in Iraq. Dillon said another leading figure was arrested when he tried to escape among a group of civilians.

Airstrikes on the city appeared to have decreased recently, apparently to allow for the evacuations. The coalition reported just five airstrikes near Raqqa on Tuesday.

The extremist group has suffered a series of major battlefield defeats in both Iraq and Syria in recent months, but has continued to stage attacks far from the front lines.

At least three suicide bombers struck outside the police headquarters in central Damascus on Wednesday, killing at least two people and wounding others, according to the Syrian interior minister.

IS claimed the attack through its Aamaq news agency. The extremist group carried out a similar attack on a Damascus police station last week that killed 17 people.

The pro-government Al-Ikhbariya TV on Wednesday showed pictures from outside the heavily guarded police headquarters, which is encircled by blast walls. The street was littered with shrapnel but there was no sign of other damage.

News from © The Associated Press, 2017