BEIRUT -- A cease-fire brokered by Russia and Turkey went into effect in war-ravaged Syria at midnight Thursday, a potential breakthrough in the six years of fighting that have left more than a quarter-million people dead and triggered a refugee crisis across Europe.

If it holds, the truce between the Syrian government and the country's mainstream rebel forces will be followed by peace talks next month in Kazakhstan, Russian President Vladimir Putin said in announcing the agreement. He described it, however, as "quite fragile" and requiring "special attention and patience."

It was not immediately clear whether the truce -- which excludes extremist factions such as the Islamic State group and al-Qaida affiliates in Syria -- was holding. It is expected at least to reduce the violence that has gripped the country, including government airstrikes.

Opposition activist Mazen al-Shami said half an hour before midnight, the situation became "very calm" in the suburbs of the capital Damascus.

He added a government offensive on the rebel-held Barada Valley northwest of Damascus that had been going on for days had stopped.

The truce had the backing of Russia and Turkey, which have been supporting opposing sides in the war, and was welcomed by Iran and the United Nations. Russia said the deal was signed by seven of Syria's major rebel factions.

Several previous cease-fires all collapsed, some in a matter of days; nevertheless, the deal this time raised hopes for a political settlement to the ruinous war in the coming months, in part because the landscape has shifted significantly.

For one thing, the tide has turned in Syrian President Bashar Assad's favor militarily over the past year, with the government retaking the city of Aleppo from the rebels just days ago. Also, Turkey, which is fighting Kurdish and Islamic militants at home, appears more willing to strike a bargain with Russia if it means protecting its borders.

"This is a different political scene, and one would expect some outcomes to emerge," said Hilal Khashan, political-science professor at the American University of Beirut. He cautioned, however, against expecting immediate results from the first round of talks.

Syrian Foreign Minister Walid al-Moallem welcomed the cease-fire agreement and said there is a "real chance" for a political settlement.

In comments made to Syrian TV, he said the Syrian government will attend the peace talks in the Kazakh capital Astana "with an open mind" but suggested it would not be willing to compromise on Assad's fate.

"Everything is negotiable except national sovereignty and the people's right to choose its leadership," he said.

Putin said the cease-fire will be guaranteed by Moscow -- Assad's chief patron and battlefield ally -- and by Turkey. Turkey is a main backer of the opposition forces, who use the country's long border with Syria to cross back and forth, and has wide influence on them.

Iran, for its part, has been one of Assad's strongest backers.