BEIRUT — A cease-fire went into effect in Syria at sunset Monday in the latest attempt led by the U.S. and Russia to bring some quiet in the 5 1/2-year civil war.

Residents and observers reported quiet in most of the country hours after the truce went into effect, though activists said airstrikes took place on contested areas around the northern city of Aleppo.

But the most powerful rebel groups have shown deep misgivings over the cease-fire deal, which was crafted without their input last weekend in Geneva between the top U.S. and Russian diplomats. Hours after it went into force, a coalition of rebel factions put out a statement that stopped short of committing to the cease-fire, a reflection of their distrust of the government.

Confusion also surrounded the cease-fire Monday as Secretary of State John Kerry said the U.S. and Russia could permit President Bashar Assad's government to launch new airstrikes against al-Qaida-linked Jabhat Fatah al-Sham, formerly the Nusra Front. The State Department quickly reversed itself.

Spokesman John Kirby said later there were no provisions under the nationwide truce for U.S.-Russian authorization of bombing missions by Assad's forces. “This is not something we could ever envision doing,” he said.

While Kirby called his boss' remarks “incorrect,” Kerry's statement reflected the general murkiness of an agreement that hasn't been presented publicly in written form. The deal came after marathon negotiations between Kerry and Russian Foreign Minister Sergey Lavrov last Friday; descriptions by the two diplomats represent the only public explanation of what was agreed to.

The U.S. had never previously spoken of approving military operations by Assad.

Later, Kerry's spokesman said a “primary purpose of this agreement, from our perspective, is to prevent the Syrian regime air force from flying or striking in any areas in which the opposition or Nusra are present.”

Once U.S.-Russian military cooperation is established, Kirby said, the focus would be to “coordinate military action between the U.S. and Russia, not for any other party.” The U.S. and Russia would target militants including the al-Qaida-linked insurgents.

Kerry implored all of Syria's warring sides to adhere to the cease-fire. He said that there has been a reduction in violence in its first hours and that it offers an opportunity for peace.

However, the al-Qaida-linked insurgents are closely allied to many rebel factions and are a powerful force in the defense of Aleppo in particular. That raises the danger that continued airstrikes will draw rebels into retaliation, eventually leading to the cease-fire's collapse.

Compounding the situation, a group of 21 rebel factions issued a statement in which they warned against targeting al-Qaida-linked militants. The statement was noncommittal about whether the groups would join the cease-fire.