The truce is the second such agreement since an Aug. 1 ceasefire deal broke down only days after going into effect.
On Friday, Moscow announced that Syrian forces would observe a new cease-fire from Saturday morning in Idlib.
It said the truce aimed "to stabilize the situation" in the anti-government bastion.
Syrian state news agency SANA on Saturday said the government agreed to the deal.
But the army "reserves the right to respond to violations" by militants and allied rebel groups, it added, citing a Syrian military source.
The head of the so-called Syrian Observatory for Human Rights said airstrikes had stopped since the agreement went into effect at 6:00 a.m. (0300 GMT).
"There are no warplanes in the sky and airstrikes have stopped," Rami Abdel-Rahman told AFP.