BEIRUT -- The U.S.-led coalition battling the Islamic State of Iraq and Syria (ISIS) said on Wednesday that it won't accept a negotiated withdrawal for hundreds of ISIS 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 ISIS militants are holding some civilians as human shields, preventing them from escaping as the fight enters its final stages for the last remaining slice of Raqqa in militant hands. 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 has called on all parties to the conflict to take all measures to protect civilians. The U.N. said September was the worst month in 2017 for civilians in Syria.

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Dillon said the Raqqa Civil Council, a local administration of Arab and Kurdish officials, was leading the discussions to ensure safe evacuation of civilians as the fight for Raqqa enters its final stages. However, it was not clear with whom the council is speaking inside Raqqa. A Kurdish-led force, the Syrian Democratic Forces, is leading the U.S.-backed battles 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 holed up in about 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 spotted in Iraq as the extremist group's power wanes in both countries. Dillon said at least another leading figure was arrested among civilians, trying to escape.

Meanwhile, Russia accused the U.S. military on Wednesday of allowing ISIS to operate with impunity "under its nose" along the Iraqi-Syrian border, suggesting American troops were letting the militants flee Iraq into Syria, where Russian forces and their Syrian allies would have to fight them.

Russia's Defence Ministry claimed about 300 ISIS militants had driven in pickup trucks from Iraq into Syria, passing through territory near a U.S. military base at Tanf on the Syrian side before trying to block a highway in the region used to supply Syrian forces.

Defence Ministry spokesman Major General Igor Konashenkov said in a statement that the U.S. should "explain about another incidence of their 'selective blindness' towards militants operating under their nose."

Konashenkov suggested the U.S. "blindness" could precipitate "an attempt to rupture the peace agreement" in place in southern Syria which has largely eased violence in the region.

Asked by the Reuters news agency about the Russian allegations, Pentagon spokesman Colonel Robert Manning said the U.S. was committed to the unconditional surrender or killing of ISIS militants.