Either way, the tempo of airstrikes by the American-led coalition against Islamic State targets around Al Bab has increased in recent days, with six strikes reported by the military on Tuesday alone, and 35 since Jan. 1.

Both pro- and anti-government forces expect to take advantage, vowing to seize Al Bab within days, fighters and supporters say.

Russia and Turkey have agreed that the pro-government forces, not the rebels, will enter the city, according to two government supporters with knowledge of the plans. Otherwise, they said, the rebels — who have pressed Al Bab on three sides for months — would have seized it long ago.

One of these people is an army soldier fighting there. The other is a pro-government informant from Al Bab, who now lives outside Syria but has relayed information to the Syrian military from his contacts in the city. Both were reached via internet chat and requested anonymity because they were not authorized to communicate with reporters.

Russia insisted on the terms for occupying Al Bab in a deal reached last month in Astana, Kazakhstan, according to the informant, who said Russia wanted pro-government forces to take the city in part to seize water facilities that could help alleviate shortages.

But it remains unclear if all rebel forces backed by Turkey in the Al Bab siege have accepted those terms. Some stepped up their assault on Wednesday and vowed to take the city first.

Allowing pro-government forces to occupy Al Bab could be humiliating for the rebels before a new round of peace talks scheduled to start on Feb. 20 in Geneva. But with shrinking options, the rebels are increasingly wedded to the wishes of their Turkish backers.