BEIRUT, Lebanon — A cease-fire between Syria’s government and the weakened rebel forces arrayed against it took effect early Friday, but within hours, violations were reported.

Still, the agreement — announced on Thursday by the Syrian government’s strongest backer, President Vladimir V. Putin of Russia — could be a turning point in a nearly six-year conflict that has killed hundreds of thousands of people and defied repeated efforts to end it.

The agreement highlighted Russia’s status as the main international player in Syria. The United States, which has supported rebel factions, welcomed the truce but played no role in brokering it.

The accord appeared shaky even before it took effect, with the parties disagreeing on issues that have sunk past peace plans, like the fate of Syria’s president, Bashar al-Assad. A spokesman for Ahrar al-Sham, one of the largest rebel groups, wrote on Twitter that his group had reservations about the agreement and had not signed it, throwing its compliance into question.

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.

In addition, the Obama administration has reduced its engagement in Syrian diplomacy, while President-elect Donald J. Trump has vowed to cut support for the rebels and to work with Mr. Putin to fight jihadists.

As the United States has stepped back, Russia has stepped forward. Mr. Assad’s forces, ground down by years of war, have become heavily dependent on Russian military support, giving Russia the leverage to push Mr. Assad toward new talks.

But after nearly six years of a war that has drawn in many foreign powers, the issues to be resolved are many, and cracks in the agreement surfaced late Thursday, before the cease-fire took effect.

Left unresolved was the future role of Mr. Assad. Many rebels have vowed to reject any deal that would leave him in power. Some even called the cease-fire a time to regroup for future battles.

“The cease-fire is a new opportunity for the revolutionaries to get their house in order and prepare for all coming possibilities to topple the regime, militarily and politically,” Ammar Sakkar, a spokesman for a prominent rebel faction, wrote on Twitter.

Also unresolved was the presence in Syria of fighters from the Lebanese militant group Hezbollah and other Shiite militias, which gave substantial support to Mr. Assad’s forces in the battle for Aleppo. On Thursday, Mevlut Cavusoglu, the Turkish foreign minister, said they should leave the country, a demand the Syrian government is likely to resist.

Excluded from the cease-fire, according to the Syrian Army, are the jihadists: the Islamic State, which controls territory in eastern Syria and across the border in Iraq; the Syrian affiliate of Al Qaeda, which is strongest in the country’s northwest; and “groups linked to them.”

How to define groups “linked” to the jihadists will be a thorny issue. The Syrian government has portrayed all of its opponents as terrorists.

The agreement was reached in the Turkish capital, Ankara, in talks that included Turkish and Russian officials and rebel representatives. It called for a cease-fire across Syria to begin at midnight. That would clear the way for peace talks in late January in Astana, the capital of Kazakhstan.

That is a far cry from Geneva, where previous talks hosted by the United Nations have been held, but the office of the United Nations special envoy for Syria, Staffan de Mistura, issued a statement cautiously welcoming the truce.

The Russian foreign minister, Sergey V. Lavrov, said he hoped Mr. Trump would join the efforts after he takes office on Jan. 20.

The Syrian government described the cease-fire as a natural follow-up to “the victories and successes that our armed forces accomplished,” a clear reference to Aleppo. Mr. Putin spoke on Thursday with Mr. Assad, who “expressed willingness to comply” with the accord, according to the Kremlin’s website.

The precise details of the deal were not clear Thursday night. Russia said seven rebel groups had signed it. Together, they hold territory in Syria’s northwest, along the border with Turkey; east of the capital, Damascus; and in the south, near Jordan’s border. Five of them are mainline rebel groups that have received covert military aid through a program run by the C.I.A. and its counterparts in allied countries. The list also includes Ahrar al-Sham, a hard-line Islamist group with close operational ties to Al Qaeda’s affiliate in Syria.

A spokesman for the group denied that it had signed the agreement.

The Obama administration, which has resisted greater involvement in what it sees as an intractable conflict, welcomed the news. “Any effort that stops the violence, saves lives and creates the conditions for renewed and productive political negotiations would be welcome,” said the State Department’s deputy spokesman, Mark C. Toner.

If the agreement holds, it may solidify Mr. Assad’s grip on the country’s western ridge and lead to a joint effort by Russia and the United States against Islamic State militants. But that is a big if, given the number of parties involved, their competing interests and the scope of the fighting.

Aside from the politics of it, many Syrians hoped the truce would stop the violence that has become part of everyday life.

“It is a way to stop the waterfall of blood that is happening in Syria,” said Obaida al-Omar, an antigovernment activist reached through Skype in a rebel-held part of Idlib Province.

He said the miserable conditions there for Syrians displaced by the war had fueled hatred of Mr. Assad’s government.

Idlib “is like a pot boiling with hatred for the regime in these terrible conditions of hunger and cold,” he said. “We hope that there are negotiations, but we hope that they don’t give up on demanding the fall of the regime.”

Mr. Putin said the cease-fire meant that Russia could draw down its forces in Syria. He said once before, in March, that the bulk of Russian forces would go home, yet combat continued.

Correction: December 29, 2016
Because of an editing error, an earlier version of this article misstated the role of Hezbollah in the Syrian conflict. It has sent fighters to battle alongside President Bashar al-Assad’s forces, not to battle against them.