The truce is the second such agreement since an August 1 ceasefire deal broke down only days after going into effect, prompting Damascus and regime ally Moscow to resume bombardment.

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

Air strikes by Damascus and Russia have killed more than 950 civilians since the end of April, according to the Britain-based Syrian Observatory for Human Rights.

The United Nations says the violence has also displaced more than 400,000 people.

On Friday, Moscow announced that Damascus government forces would observe a new ceasefire from Saturday morning in Idlib, saying the truce aimed "to stabilise the situation" in the anti-government bastion.

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

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

Observatory head Rami Abdel Rahman said air strikes had stopped since the agreement went into effect at 6:00 am (0300 GMT).

"There are no warplanes in the sky and air strikes have stopped," he told AFP.

Clashes between regime loyalists and insurgents on the edges of the anti-government bastion have also ceased, he said.

However, sporadic artillery and rocket fire continued despite the deal, he added.

- Medical centre attacked -

Taking advantage of the truce, Ahmed Ibrahim returned Saturday to his southern Idlib village of Maar Shurin to collect a few belongings, insisting he will not stay long.

"The regime, like usual, will violate the ceasefire and won't keep its promises," Ibrahim, who fled his hometown 15 days ago and moved north towards the border with Turkey, told AFP.

Saturday's truce is the latest attempt to avert a full-blown offensive, which the UN has said would result in one of the worst humanitarian "nightmares" in Syria's eight-year conflict.

Only a few hours before it went into effect, a Russian air strike hit a health facility in Aleppo's western countryside, the Observatory said.

It said the attack near the town of Urum al-Kubra after midnight wounded several health workers and left the medical centre out of service.

The UN has said 43 hospitals and clinics and 87 educational facilities have been impacted by fighting since April.

"The attacks we have seen on health facilities, educational facilities and water points is one of the highest in the world," Panos Moumtzis, the UN's Syria humanitarian chief, told AFP.

"This is unacceptable."

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

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

"The average family in Idlib has been displaced five times," Moumtzis said, adding that displacement is among the largest recorded by the UN during Syria's war.

On Friday, before Moscow announced the ceasefire, hundreds of Syrian protesters rallied on the Syrian side of the Turkish border, demanding Ankara help stop deadly bombardment by Damascus.

- 'Operational pause' -

The Idlib region is home to some three million people, nearly half of whom have been displaced from other parts of Syria.

Most of Idlib province and parts of neighbouring Aleppo and Latakia provinces are controlled by Hayat Tahrir al-Sham, a jihadist group led by Syria's former Al-Qaeda affiliate.

Other rebels and jihadists are also present.

It is supposed to be protected from a massive government offensive by a Turkish-Russian deal struck in September 2018 that was never fully implemented as jihadists refused to withdraw from a planned demilitarised cordon.

"Russia and the Syrian government may be willing to give Turkey another opportunity to implement the terms of its September 2018 bilateral agreement with Russia," said Sam Heller of the International Crisis Group.

"Alternately, this ceasefire may just be an operational pause for Damascus and Moscow to consolidate their territorial gains and prepare for the next phase of their offensive," the Syria expert added.

Syrian President Bashar al-Assad, whose forces control around 60 per cent 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.