Updated

Confusion reigned Monday over Syria's new cease-fire as Secretary of State John Kerry said the United States and Russia could permit President Bashar Assad's government to launch new airstrikes against Al Qaeda-linked militants. 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.

Kerry's comments at a news conference were the closest any American official had come to suggesting indirect U.S. cooperation with Assad since the civil war started in 2011. President Barack Obama called on Assad to leave power more than five years ago; the U.S. blames the Syrian leader for a war that has killed perhaps a half-million people.

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 a marathon negotiation 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.

Under the truce that went into effect at sundown Monday, Assad's forces are no longer supposed to bomb Syria's opposition, Kerry said.

If calm holds for seven days, the U.S. and Russia would then cooperate on how to jointly combat the Al Qaeda-linked group formerly called the Nusra Front and now known as Jabhat Fatah al-Sham. But the two powers also could approve Syrian combat missions against the group, he said.

"Assad is not supposed to be bombing the opposition, because there is a cease-fire," Kerry told journalists at the State Department. "Now he is allowed ... to target Nusra. But that will be on strikes that are agreed upon with Russia and the United States in order to go after them."

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."

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

But in Aleppo, the northern city that has emerged as the epicenter of the fighting, opposition media activist Mahmoud Raslan said government helicopters dropped crude barrel bombs on a contested neighborhood. A doctor reported heavy shelling along the Castello road where supplies are expected to go through to reach rebel-held parts of Aleppo. The doctor spoke on condition of anonymity for fear of retribution.

Various militants backed by the U.S. and its Arab allies coordinate and even sometimes fight alongside Al Qaeda-linked militants. Many Syrian and Russian operations have struck what U.S. officials describe as "moderate" forces that are "marbled" with the militants.

The new cease-fire is supposed to end such ambiguities, and Washington has been urging rebel groups to break ranks with extremists.