State news agency SANA reported Thursday that the government had agreed to a truce in the northwestern region where air strike and shelling by the government and its Russian ally has killed hundreds sof people since April, many of them civilians.
Citing a military source, SANA said a ceasefire would go into effect on Thursday evening but was conditional on the implementation of a Turkish-Russian deal to enforce a buffer zone encircling the region.
"A cautious calm has reigned since just before midnight (2100 GMT)," Rami Abdel Rahman, head of Britain-based monitoring group, the Syrian Observatory for Human Rights, told AFP.
He said that Syrian and Russia aircraft were no longer seen flying over Idlib while "fighting on the ground had also ceased on all fronts in the past few hours".
An AFP correspondent reported that an early warning system known as Sentry registered the last air raids on the Idlib province town of Khan Sheikhun just two minutes before the truce took effect at midnight.
The Sentry programme uses human observers and a network of sensors to compute a predicted impact location when Syrian or allied warplanes take off.
The resulting estimate can then trigger air raid sirens near the target zone and send warnings to mobile phone applications, giving residents more time to take cover.
Most of Idlib province and parts of Hama, Aleppo and Latakia are controlled by Hayat Tahrir al-Sham, a jihadist alliance led by Syria's former Al-Qaeda affiliate.
The region was supposed to have been spared from a threatened government offensive last year by a deal struck by Ankara and Moscow in the Russian resort of Sochi in September.
But the agreement has faltered and Syrian forces have launched an intensified bombardment of the region with Russian support since late April.
According to the Observatory, some 790 civilians have been killed in the past three months along with 1,000 jihadists nd other rebels and around 900 pro-government fighters.
The UN says that more than 400,000 people have been displaced by the fighting.
SANA's announcement of a ceasefire came as talks resumed in Kazakhstan between rebel backer Turkey and regime allies Russia and Iran.
The Syrian conflict has killed more than 370,000 people and driven millions from their homes since it started with the brutal repression of anti-government protests in 2011.