Heightened regime and Russian bombardment has hit the jihadist-held province of Idlib since mid-December, as regime forces make steady advances on the ground despite an August ceasefire and UN calls for a de-escalation.
Nearly 80 civilians have been killed by air strikes and artillery attacks over the same period, according to the Syrian Observatory for Human Rights, which estimates that more than 40,000 people have been displaced in recent weeks.
"These attacks should come to an end immediately," Turkey said Tuesday after sending a delegation to Moscow to discuss the flare-up.
Presidential spokesman Ibrahim Kalin said Ankara was pressing for a new ceasefire to replace the August agreement.
The French foreign ministry called for "an immediate de-escalation".
"The military offensive by the Damascus regime and its allies is worsening the humanitarian crisis" in Idlib, it said in a statement.
Tuesday's strikes targeted the village of Jubass near the town of Saraqeb in southern Idlib, killing civilians sheltering in a school and nearby, said the Britain-based Observatory, which has a network of contacts across Syria.
The site of the attack was strewn with destroyed tents and smoking debris, said an AFP correspondent there.
Standing amid the wreckage, Hassan -- who has been living in the informal settlement -- said he was trying to leave when the attack happened.
"As we were packing the car an air strike hit us," he said.
- Ground assault -
The United Nations children's agency UNICEF on Tuesday condemned the violence in Syria's last major opposition bastion.
"Children are bearing the brunt of intensifying violence in northwest Syria," it said in a statement.
The escalation came as Damascus loyalists advanced on the ground.
Since Thursday, regime forces have taken control of dozens of towns and villages in southern Idlib following battles with jihadists.
The clashes have killed 260 fighters on both sides, according to the Observatory.
Regime forces are now less than four kilometres (two miles) from the strategic city of Maaret al-Numan, Observatory chief Rami Abdel Rahman told AFP.
In a statement on Tuesday, the Syrian army said it has seized 320 square kilometres from its rivals in recent days.
It pledged to continue its push until it recaptures all of Idlib, calling on civilians to exit areas under jihadist control.
But fearing further regime advances from the south, thousands of Maaret al-Numan's residents have fled northwards towards the Turkish border.
"I didn't expect to have to leave," said Abu Ahmad, poking his head out of the pick-up truck driving him and his family towards a camp for the displaced.
"This is my home, this is where I grew up," the father of ten told AFP.
- 'Life-saving assistance' -
Idlib is dominated by the country's former Al-Qaeda affiliate, Hayat Tahrir al-Sham.
The head of the group urged jihadists and allied rebels Tuesday to head to the frontlines and battle "the Russian occupiers" and the regime.
Their "ferocious" campaign "requires us to exert more effort," said HTS chief Abu Mohammed al-Jolani in a statement by his group's propaganda arm.
The region hosts some three million people, including many displaced by years of violence in other parts of Syria.
The Damascus regime, which now controls 70 percent of Syria, has repeatedly vowed to take back the area.
Backed by Moscow, Damascus launched a blistering offensive against Idlib in April, killing around 1,000 civilians and displacing more than 400,000 people.
Despite a ceasefire announced in August, the bombardment has continued, killing hundreds of civilians and fighters.
The latest spike in violence comes after Russia and China on Friday vetoed a UN Security Council resolution that would have extended for a year cross-border aid deliveries to four million Syrians, many of them in Idlib.
The move raised fears that vital UN-funded assistance could stop entering opposition-held parts of Syria from January unless an alternative agreement is reached.
France on Tuesday called on member states of the UN Security Council, namely Russia and China, to renew the resolution.
"It is more imperative than ever for the United Nations to maintain the most direct and effective access to populations in need through preserving cross-border assistance," the foreign ministry said in a statement.
To help civilians in Syria's northwest, it said it has released additional emergency aid of just over five million euros to the UN cross-border humanitarian fund based in the Turkish city of Gaziantep.
Syria's war has killed over 370,000 people and displaced millions since beginning in 2011 with the brutal repression of anti-government protests.
