Russian President Vladimir Putin says the Syrian government and its opponents have signed an agreement on a nationwide cease-fire and a declaration expressing willingness to begin peace talks.
Putin said on December 29 that a document outlining measures to implement the cease-fire was also signed.
The truce due to take effect at midnight local time would be the first nationwide halt in fighting since a weeklong cease-fire in September that collapsed after several incidents of violence.
The Turkey-based Syrian National Coalition, a major Syrian opposition group, confirmed its support for the truce.
The Syrian Army also confirmed the planned nationwide halt to fighting.
"It has just been reported that an event occurred several hours ago that we had not just waited a long time for but have worked to speed up. Three documents have been signed," Putin said in televised remarks at a meeting with his foreign and defense ministers.
He said that that "all the agreements that have been reached are very fragile and require special attention and patience, a professional approach toward these issues, and constant contact with our partners."
The announcement came days after forces of Syrian President Bashar al-Assad's government took full control of the northern city of Aleppo, forcing out rebels who had held the eastern part of the city since 2012. The key victory for Assad followed an intense offensive that drew condemnation from Western governments, human rights groups, and Syrian activists.
The announcement was welcomed by the United States, which had been pointedly left out of these latest negotiations despite being an ally of Turkey.
"We hope [the cease-fire] will be implemented fully and respected by all parties," State Department spokesman Mark Toner said in a statement. "Any effort that stops the violence, saves lives, and creates the conditions for renewed and productive political negotiations would be welcome."
Russian Defense Minister Sergei Shoigu said that Russia and Turkey would be guarantors of the cease-fire, which he said included more than 60,000 rebels and that "these groups control most of central and northern Syria."
The AFP news agency quoted a spokesman for the National Coalition, Ahmed Ramadan, as saying the group "expresses support for the agreement and urges all parties to abide by it."
Ramadan said that key rebel groups including the powerful Ahrar al-Sham and Army of Islam factions had signed the cease-fire deal, though there was no immediate confirmation from rebel officials.
Russia To Continue 'Fighting Terrorism'
The Syrian Army said the cease-fire did not apply to combat against the extremist group Islamic State (IS) and the Al-Nusra Front, which now calls itself the Fateh al-Sham Front and says it is no longer affiliated with Al-Qaeda.
A spokesman for the Free Syrian Army, Osama Abu Zaid, said the agreement also excluded the Kurdish People's Protection Units (YPG) militia and that the Syrian Kurdish Democratic Union Party (PYD) political wing would not be part of the upcoming talks to be held in Astana, the capital of Kazakhstan.
That aspect of the deal was a direct concession to Turkey, which fears that a stronger Kurdish fighting force in Syria will further embolden Turkish Kurds.
Ankara supports the Free Syrian Army, a loose alliance of moderate rebel factions, some of which it is backing in operations in northern Syria designed to sweep IS and Syrian Kurdish fighters from its southern border.
Putin said that the Russian military would scale down its presence in Syria, but he didn't say how many troops and weapons would be withdrawn.
Putin said Russia will continue "fighting international terrorism in Syria" and supporting the Assad government.
The Russian military will maintain its presence at both an air base in Syria's coastal province of Latakia and the naval facility in the Syrian port of Tartus, Putin added.
Putin spoke hours after Turkish Foreign Minister Mevlut Cavusoglu said Turkey and Russia had prepared a cease-fire agreement and a document outlining a political solution to the conflict. He did not say where the agreements had been signed.
Russia and Turkey have backed opposing sides in the war, which has killed more than 250,000 people in Syria since it erupted following a deadly government crackdown on pro-democracy protests in March 2011.
It was unclear if Assad had made any public announcement, but the Kremlin released a statement saying Putin and Assad had spoken by telephone, and that Assad was committed to abiding by the cease-fire.
Moscow has given crucial diplomatic and military support to Assad and helped avert his government's possible defeat by launching a campaign of air strikes in September 2015, while Turkey has supported rebels seeking Assad's ouster.
However, Ankara and Moscow have started to cooperate more closely on Syria in recent months.
Last week, Putin said that Turkey, Russia, and Iran were expected to take part in proposed peace talks to be hosted by Kazakhstan.
A few hours after Putin's announcement, Turkish President Recep Tayyip Erdogan's spokesman said he and Putin discussed the cease-fire and the planned talks in the Kazakh capital, Astana, during a telephone conversation on December 29.
Erdogan later told a news conference in Ankara that the cease-fire was a historic opportunity to end the conflict.
"This is a window of opportunity that has been opened and should not be squandered," he said.
"The fight against terror groups, including [Islamic State], will continue with determination until the security of our citizens is assured," he said.
During the meeting with Putin, Russian Foreign Minister Sergei Lavrov said on December 29 that Egypt will be invited to join the process and that Saudi Arabia, Qatar, and Jordan could eventually join as well.
Lavrov also said he hopes that after U.S. President-elect Donald Trump takes office, his administration "will also join the efforts in order to channel this work in one direction based on friendly and collective cooperation."
President Barack Obama's administration worked closely with Russia in efforts to end the fighting in Syria, where it leads a coalition combating IS militants and has accused Moscow of failing to deliver on previous cease-fire agreements.
The deal comes after a localized truce, also brokered by Russia and Turkey, that set the stage for the removal of civilians and rebels from the eastern part of Aleppo earlier this month.
Putin's announcement also came amid expectation that Obama's administration will impose new sanctions on Russia over allegations that it hacked and leaked e-mails of Democratic Party organizations and operatives.
The CIA has concluded that effort was an attempt to help Trump -- who has said he would seek to improve badly strained relations with Moscow -- defeat Democrat Hillary Clinton in the November 8 election.