Airstrikes on Syria's northwestern Idlib region stopped on Saturday, a war monitor said, after the regime agreed to a Russian-backed ceasefire following four months of deadly bombardment.

"There are no warplanes in the sky and airstrikes have stopped," said Rami Abdul Rahman, the head of the Britain-based Syrian Observatory for Human Rights.

Clashes between regime loyalists and insurgents on the edges of the anti-government bastion have also ceased after a unilateral ceasefire by Syrian regime forces went into effect at 6 a.m. (0300 GMT), he said.

Artillery and rocket attacks continued however despite the deal, he added.

Syrian regime ally Russia on Friday announced that Damascus government forces would observe a new ceasefire from Saturday morning in Idlib.

It said the truce aimed "to stabilize the situation" in Idlib, one of the last holdout of opposition to forces of Syrian President Bashar al-Assad.

The Syrian government agreed to the deal, state news agency SANA said.

But the army "reserves the right to respond to violations" by terrorists and allied rebel groups, it added, citing a Syrian military source.

The truce is the latest Russian-led effort to avert what the United Nations has described as one of the worst humanitarian "nightmares" in Syria's eight-year conflict.

Only a few hours before it went into effect, the Observatory reported a Russian airstrike on a health facility in Aleppo's western countryside.

It said the attack after midnight wounded several health workers and put the medical center out of service.

The U.N. has said that 43 health facilities and 87 educational facilities have been impacted by fighting since April.

The Idlib region is home to some 3 million people.

Most of the province and parts of neighboring Aleppo and Latakia provinces are controlled by Hayat Tahrir al-Sham, a terrorist group led by Syria's former al-Qaida affiliate.

Russia-backed regime forces have been pressing an offensive against the region despite a deal with moderate opposition-backer Turkey in September last year to protect the area.

Stepped up bombardment by Damascus and Moscow since the end of April has displaced 400,000, the U.N. says, and killed more than 950 civilians, according to the Observatory.

The flare-up has emptied entire towns and villages in northern Hama and southern Idlib of their residents, according to the U.N.

Almost half of the displaced are living in camps, reception centers or in the open air, it added.

Earlier this month, Damascus scrapped a similar truce agreement only three days after it went into effect, accusing rebels and terrorists of targeting a Russian airbase.

Assad, who now controls around 60 percent of the country, has vowed to reclaim the rest, including Idlib.

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.