Apparently, the group still had not signed when the cease-fire began at midnight. The Syrian Observatory for Human Rights, which tracks the conflict, said in a statement on Twitter early Friday that rebels had breached the cease-fire in Hama Province. A spokesman for the rebel group Jaish al-Nasr told Reuters that government forces had violated the truce, shelling areas in Atshan and Skeik, villages in Idlib Province, which borders Hama.

Even before the new clashes were reported, Ahrar al-Sham’s status and other issues had left many analysts skeptical.

“It looks like the Russians are trying to jam this through and make it happen through pure positive thinking and momentum,” said Sam Heller, a fellow at the Century Foundation who studies Syria. “All indications are that there are still major unresolved questions and issues that will sabotage its implementation.”

Many past efforts to quell the fighting, including two agreements this year between Russia and the United States, have failed, and Mr. Putin himself called the deal fragile. But in recent months, major shifts have occurred in the war and among the foreign powers embroiled in it, creating an opening for the new agreement.

The rebels’ loss of eastern Aleppo this month was a major blow to their movement, leaving them without footholds in any of Syria’s largest cities. That has made it harder for their foreign backers to see them as a realistic alternative to Mr. Assad. Even Turkey, which is a longtime supporter of the opposition and is supposed to ensure rebel compliance with the cease-fire, has backed away from its demand that Mr. Assad step down.