The "regime of calm" was announced on Sept. 12 following a ceasefire brokered by the United States and Russia.
Both militants and the government have accused each other of numerous violations of the ceasefire, the second this year.
In its statement announcing the "calm", the army had said it would expire at 11:59 p.m. (2059 GMT) on Sunday, Reuters reported.
The development comes as US-led coalition jets on Saturday bombed Syrian government forces’ positions near the eastern city of Deir ez-Zor, killing 90 troops and "paving the way" for Daesh militants, according to the Syrian Army General Command.
The Syrian General Command has called the bombing a “serious and blatant aggression” against Syrian forces, and said it was "conclusive evidence" that the US and its allies support Daesh militants.
Russia's Foreign Ministry said Sunday the airstrikes threatened the implementation of a ceasefire plan for Syria and urged Washington to thoroughly investigate the accident.