US dismisses negotiated withdrawal of Islamic State fighters from Syria's Raqqa
But Dillon added that discussions about the fate of IS 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 four square kilometres of Raqqa, including in the city's stadium and a hospital, he said.
The stadium is believed to be used by the militants as a weapons warehouse and a prison, while the hospital is one of their major headquarters.
The Raqqa Civil Council, a local administration of Arab and Kurdish officials, is leading the discussions to ensure the safe evacuation of civilians.
However, it was not clear with whom the council is speaking with inside Raqqa.
In the past three weeks up to 15 militants including a senior leader have surrendered in Raqqa, Dillion said, while another leading figure was arrested as he tried to escape among a group of civilians.
Since IS captured Raqqa in 2014 tens of thousands of civilians have fled the city.
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.