The U.S. is reviewing a Russian-backed plan that is aimed a lowering the level of violence but establishing "de-confliction zones" in Syria, Defense Secretary Jim Mattis said Monday.

"All wars eventually come to an end, and we've been looking for a long time how to bring this one to an end," Mattis told reporters traveling with him to a counter-ISIS meeting in Copenhagen, Denmark.

"We'll look at the proposal," Mattis said "[to] see if it can work."

Mattis said many details of the safe zones proposed last week after peace talks in Kazakhstan are still unclear, including the specific borders of the area, who would be protected, who would be kept out, which parties agree and who would be the guarantor of safety.

"The devil's always in the details, right?" Mattis said according to a transcript provided by the Pentagon. "There's a lot of details to be worked out is what I'd say."

Russia has said its proposal for safe zones would include a ban on U.S. and coalition aircraft flying over the area, which are mostly in the east where the U.S.-led coalition doesn't fly.

Mattis stopped short of saying whether the Russian no-fly zones would be respected if ISIS was being targeted.

"I think the international community is united in a sense of wanting to see ISIS put on its back foot," Mattis told reporters, implying the Russian proposal would not affect the effort to defeat the Islamic State in the western part of Syria.

"The fight against ISIS will go on," Mattis said.

Mattis arrived in Copenhagen Monday to attend a meeting of more than a dozen countries that are taking a primary role in the U.S.-led counter-ISIS coalition.

"We're going to sit down, take stock of where we're at. We're going to look to the future, determine what more is needed, if anything," Mattis said.

Mattis said the meeting would begin with a progress report, but he said there is little doubt ISIS is losing.

"They're on their back foot in Syria. They're on their back foot in Iraq. They've lost a lot of ground," Mattis said.