The Syrian army says it has begun implementing a US-Russian ceasefire, but the country's most powerful insurgent groups have not yet said whether they will abide by it.
The Syrian government and its main allies, Russia and Iran, say they will abide by the week-long truce, which was set to begin at 7pm local time on Monday.
Half an hour before the truce came into effect, violence was reported in several areas throughout Syria.
The deal, announced last week by Washington and Moscow, calls for a halt to fighting between the US-backed opposition and the Russian-allied Syrian government.
If the truce holds for a week, the United States and Russia would begin intelligence sharing and target coordination against the Islamic State group and al Qaida-linked militants.