For example, if the bombing of Nusra sites kills civilians — almost inevitable given the proximity of militant extremist groups and civilians — there are bound to be accusations about who is responsible. Pentagon officials are also concerned that Russia will use the targeting data to learn more about how American forces identify and attack targets, at a time when forces from the two countries are often near one another around Europe.

Mr. Lavrov took a few shots at the United States even while celebrating the arrangement, denouncing “arrogant sanctions” levied against Russia for its annexation of Crimea two years ago.

Late on Friday, he suggested the United States could not decide about the deal, sending pizza and vodka to reporters to ease the wait, and coming by to joke about how long it took Mr. Obama and his team to make decisions.

But for Mr. Kerry, reaching this deal has become a personal mission, one that at times put him in conflict with the White House. He has pressed for a stronger military commitment in Syria and support for some opposition groups, along with a series of more aggressive covert actions, according to administration officials. Mr. Obama has been reluctant, as have others in the White House who fear that, even if they could engineer a transition in Syria, it could create a power vacuum that Iran, Russia and militant terrorist groups could exploit.

The American skepticism is partly rooted in the failed agreement for a cessation of hostilities reached with the Russians in February. That arrangement collapsed weeks later when Russia moved heavy artillery into the Aleppo area to help Mr. Assad’s forces.

Among the Syrians, the latest plan was greeted with wariness, particularly from armed opposition groups and their supporters, who, broadly speaking, have come to believe that the United States has lost interest in ousting Mr. Assad, and is willing to see them wiped out.

It is a measure of how little trust the Syrians have in the international community — especially after the short-lived cease-fire in February — that initial reactions were lukewarm, even though the deal holds out the possibility of at least a temporary calming of the violence.