U.S. Defense Secretary Jim Mattis said that the United States would closely examine the Russian proposal to create several so-called safe zones in Syria aimed at reducing violence in the conflict-torn country.
However, speaking to correspondents on May 8, Mattis warned that the plan poses many unanswered questions.
Mattis said the borders of the proposed cease-fire areas are still being resolved, although the general locations are "well understood."
"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," he said.
The plan was proposed by Russia, Syrian President Bashar al-Assad's key ally, with the support of Turkey, which backs the opposition. Iran, Assad's other major ally, also backed it.
The plan says four de-escalation zones would be established in Syria for a period of six months, which could be extended if the three signatory countries agreed.It also includes creating conditions for humanitarian access, medical assistance, and the return of refugees to their homes.
The plan, however, does not cover areas controlled by Islamic State (IS) militants, leaving the United States and its allies free to continue the campaign to retake IS-held territory.
The United States is not part of the deal, and the Syrian government and opposition haven't signed up to the agreement. But Syrian Foreign Minister Walid al-Moualem said on May 8 that Damascus would abide by the terms of the plan so long as rebels also observed it.
Mattis said that Washington owes it to the people of Syria "to at least examine [the plan] very, very carefully."
Mattis noted that chairman of the U.S. Joint Chiefs of Staff, Joseph Dunford, spoke about the zones with his Russian counterpart, Valery Gerasimov on May 7.
"This subject was brought up, but there's a lot of details to be worked out," Mattis said.
The deal is the most serious initiative to reduce violence and shore up a cease-fire first declared in December, since western states accused Damascus of a chemical attack in early April on rebel-held Idlib province.