Defense Secretary James Mattis said the United States is reviewing a Russian proposal to create “safe zones” in Syria, but the plan leaves many questions unanswered, The Associated Press reported.

Mattis said on Monday that the borders of the proposed cease-fire areas are still being worked out, and it's not known what impact the plan could have on the U.S.-led fight against Islamic State in Iraq and Syria (ISIS).

“It's all in process right now,” Mattis told reporters traveling with him to Copenhagen, Denmark, according to the AP. “Who is going to be ensuring they're safe? Who is signing up for it? Who is specifically to be kept out of them? All these details are to be worked out, and we're engaged.”

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Russia, Turkey and Iran agreed to a cease-fire between Syrian government and opposition forces in four undecided locations. The cease-fire would allow humanitarian aid to access hard-to-reach areas in Syria, and calls for refugees to be allowed to return to the safe zones and services and infrastructure to be restored.

The United States is not part of the agreement, and the Syrian government and the opposition haven't signed on to the deal, the news service reported.

Capt. Jeff Davis, a Defense Department spokesman, told reporters at the Pentagon on Friday that the cease-fire zones are not in areas controlled by ISIS fighters, which leaves the U.S. and its allies free to continue the campaign to retake territory held by the terror group.

“We have not changed or altered our operations in any way,” Davis said.

Mattis, meanwhile, said on Monday the Pentagon will look at the cease-fire proposal to see if it can work.

“Will it affect the fight against ISIS? I think the international community is united in the sense of wanting to see ISIS put on its back foot,” he told reporters, according to the AP.

The U.S. owes it “to the situation there, the people there to at least examine it very, very carefully,” Mattis added.

Mattis is in Copenhagen to attend a meeting of the anti-ISIS coalition. He will also travel to Lithuania and to a conference in London.