Members of the Syrian Arab Red Crescent carry a person reportedly killed following an airstrike in the rebel-held city of Douma in Eastern Ghouta, on Feb. 26, 2016. (Abd Doumany/AFP/Getty Images)

Guns fell silent for the first time in years in parts of civil-war-racked Syria early Saturday morning, as a cease-fire brokered by the United States and Russia went into effect at midnight, Damascus time.

The deadline came as the U.N. Security Council, meeting in New York, voted unanimously to approve the agreement to halt the brutal conflict that has left more than a quarter-million people dead, sent millions fleeing the country, and facilitated the spread of the Islamic State in Syria, Iraq and far beyond.

“Let us pray that this works,” U.N. envoy Staffan de Mistura said in Geneva, after meeting with representatives of the countries sponsoring the accord. “Because, frankly, this is the best opportunity we can imagine that the Syrian people have had over the last five years . . . to see . . . something related to peace.”

Among the many difficulties will be monitoring what the deal’s sponsors and participants call a “cessation of hostilities.” The United States and Russia, as co-chairs of a task force that includes 15 other governments supporting either the government of Syrian President Bashar al-Assad or the armed opposition fighting against it, are primarily responsible for verifying breaches and containing them from afar.

The two countries have established separate operations centers to investigate and adjudicate violation reports made to them by the parties on the ground or passed on by U.N. offices in Geneva and Damascus, the Syrian capital. These centers are in Washington and Amman, Jordan, for the United States; and Moscow and Latakia, Syria, for Russia. A hotline also has been established between the United States and Russia.

The United States, Russia and other powers came to an agreement on a ‘cessation of hostilities’ in Syria, but the deal was met with caution and skepticism. (Jason Aldag,Ishaan Tharoor/The Washington Post)

The United Nations has its own contacts on the ground, and “Russians and Americans do have their own antennas, which have been raised,” de Mistura said without offering further explanation of how the monitoring will be conducted. “The system needs to be given a case to be tested.”

Any military response to a breach should be a “last resort,” he said, and “proportionate” to the initial offense. “We should not be surprised by breaches. What we need is to make sure it is contained.”

The United States and its allies in Europe and the region who are supporting various opposition groups will be responsible for reining in those groups’ combatants, presumably through diplomatic force or threats to withhold aid. Russia, which has been steadily bombing Assad’s opponents for months, is responsible for stopping its own attacks, those by Syrian government air and ground forces, and those by Iran-backed Shiite militias acting on Assad’s behalf.

After a day in which warring groups clawed for last-minute advantage ahead of the deadline, de Mistura said he received a report, at three minutes past midnight, that “suddenly both Darayya and Damascus” had “calmed down.” One report of fighting was being investigated and another was deemed “propaganda,” he said. Darayya, which has been bombarded by the Syrian air force, is a suburb southwest of Damascus.

There were scattered but persistent reports of artillery bombardments and small-arms fire in several parts of the country, but it was difficult to know whether they represented a deliberate attempt to defy the agreement or the winding down of conflicts that have raged unchecked for years.

A Damascus resident confirmed the silence in Darayya. In the town of Kafr Hamra, along the last rebel supply line into the northern city of Aleppo, activists said that three families were buried under the rubble of their homes by a Russian airstrike a few minutes before midnight. But there were no reports of further strikes after the deadline.

Ahmad al-Masalmeh, an activist in the southern city of Daraa, said intense fighting across southern Syria stopped at midnight, the Associated Press reported.

De Mistura said that if the cease-fire holds and a separate agreement gains steam to deliver humanitarian aid to areas that have been cut off from food and medicine by the fighting, he would call on the Syrian government and the opposition to restart peace talks March 7.

“There will be no shortage of attempts to undermine” the truce, he said in a video briefing to the Security Council. De Mistura plans to meet with the task force Saturday to assess early results.

In a statement Friday, the main Syrian opposition umbrella group, known as the High Negotiations Committee, said that 97 opposition factions would observe the agreement for two weeks to gauge its effectiveness. The Assad government also issued a statement of compliance.

Under the agreement, reports of possible violations are to be transmitted by combatants to the outside operations centers via email, text message or telephone, leading to possible delays in adjudicating reported violations.

The agreement excludes the Islamic State and forces of Jabhat al-Nusra, al-Qaeda’s affiliate whose fighters are interwoven with opposition groups in the Syrian northwest around Aleppo. Bombing campaigns by the United States will continue against the Islamic State, while Russia will continue to strike the Islamic State and Jabhat al-Nusra.

Washington and Moscow — which are barely speaking to each other outside of the Syria negotiations — exchanged maps on Friday delineating where they think the excluded forces are located. In an audio statement, Jabhat al-Nusra leader Abu Mohamad al-Golani, a nom de guerre, urged fighters to “intensify strikes” against government forces.

The People’s Protection Units, or YPG, a Syrian Kurdish group allied with the United States, is not part of the agreement. Turkey, which considers the YPG a terrorist organization, has been shelling it across the border in the confused northwestern Syrian battlefield and said it does not intend to stop.

Naylor reported from Beirut. Liz Sly in Gaziantep, Turkey, contributed to this report.

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