A nationwide ceasefire in Syria brokered by the United States and Russia came into effect at 7 p.m. (1600 GMT) on Monday, the second attempt this year by Washington and Moscow to halt the five-year-long civil war.
The Syrian army said a seven-day "regime of calm" would be applied across Syria, and it reserved the right to respond using all forms of firepower to any violation by "armed groups".
Rebel groups fighting to topple President Bashar al-Assad have yet to publicly declare whether they will respect the ceasefire, but rebel sources have indicated they will.
Russia is a major backer of Assad, while the United States supports some of the rebel groups fighting to topple him, as does Turkey, which has stated its support for the agreement.
The agreement's initial aims include allowing humanitarian access and joint US-Russian targeting of jihadist groups, which are not covered by the agreement. These include Islamic State and al Qaeda's former Syria branch, known as the Nusra Front until it changed its name and cut ties to al Qaeda in July.