As reported by Censor.NET citing Reuters, it is the second attempt this year by Washington and Moscow to halt the five-year-old civil war.
The Syrian army announced the truce at 7 p.m. (1600 GMT), the moment it took effect, saying the seven-day "regime of calm" would be applied across Syria. It reserved the right to respond with all forms of firepower to any violation by "armed groups".
Rebel groups fighting to topple President Bashar al-Assad had decided to respect the ceasefire, while expressing their deep reservations about the overall agreement, said Zakaria Malahifji of the Aleppo-based rebel faction Fastaqim.
"Regarding a truce, a cease-fire, the delivery of aid, this is a moral question and there is no debate around this, we absolutely welcome this, but there are other articles around which there are reservations," he told Reuters.
Combatant sources on both sides said calm was prevailing in the first hours of the cease-fire. The Syrian Observatory for Human Rights, a Britain-based monitoring group, shared that assessment.
The agreement's initial aims include allowing humanitarian access and joint U.S.-Russian targeting of jihadist groups, which are not covered by the agreement.
The agreement comes at a time when Assad's position on the battlefield is stronger than it has been since the earliest months of the war, thanks to Russian and Iranian military support. Hundreds of thousands of Syrians have been killed in the conflict and 11 million made homeless in the world's worst refugee crisis.
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