At least 15 civilians were killed on Sunday by government air strikes in north-western Syria, a war monitored said as the United Nations voiced concern about an escalation in fighting and air strikes in Idlib province.
The raids hit a string of towns and villages in Idlib, which is almost entirely controlled by extremist groups and hardline rebels.
Ten people, including four children, were killed in the town of Taftanaz, said the UK-based Syrian Observatory for Human Rights. Five other civilians were killed in other towns.
Observatory chief Rami Abdel Rahman said strikes had also hit near a children’s hospital, which was forced to shut down.
The strike came a day after militants from Al Qaeda's former Syrian affiliate attacked the villages of Fuaa and Kafraya, which are held by forces loyal to Syrian President Bashar Al Assad.
Late on Saturday, Hayat Tahrir Al Sham (HTS) and allied fighters shelled Fuaa and Kafraya heavily and clashed with local fighters.
"This is the fiercest attack in around three years," Mr Abdel Rahman said, adding that Sunday's deadly air strikes were in response to the extremists' assault.
Six Syrian pro-regime fighters and at least three from HTS were killed in the fighting, which continued on Sunday.
Fuaa and Kafraya are the only two places in Syria currently designated by the UN as besieged after the government recaptured the Yarmouk Palestinian camp in southern Damascus.
The villages are home to an estimated 8,100 people, most of them Shiite Muslims.
They came under siege in 2015, when rebels ousted regime forces from the vast majority of Idlib province.
Meanwhile, UN Secretary-General Antonio Guterres called for an investigation of air strikes believed to have been carried out by Russian jets in Syria that killed dozens, including children.
The attack on the night of June 7 targeted the village of Zardana in Idlib province and left 44 dead, including six children, according to the Observatory.
In a statement, Mr Guterres expressed "deep concern" about the strikes and called for a "full investigation into the attacks, especially allegations that there was also a second strike targeting first responders, to establish accountability”.
He recalled that Idlib is part of the de-escalation agreement for Syria reached between Turkey, Russia and Iran, and urged those guarantors to uphold their commitment.
On Monday, the UN regional humanitarian co-ordinator for Syria, Panos Moumtzis called on major powers to broker a negotiated settlement to end the war and avoid a bloodbath in Idlib.
"We worry about 2.5 million people becoming displaced towards Turkey. There is no other location to move them to [in Syria]," he told a news briefing in Geneva.
According to the UN, more than 920,000 people have been displaced in Syria this year, the highest level since the start of conflict in 2011.
HTS and its allies control about 60 per cent of Idlib. Its local rivals hold about a third and the regime controls about 10 per cent, according to the Observatory.
Since Russia intervened in support of the Syrian regime in 2015, the government has regained control of about half of the country.
More than 350,000 people have been killed in the Syrian war since it started in 2011 with the brutal repression of anti-government protests.