‘Safe-zone’ in northwestern Syria has come under intense aerial attacks and shelling since Tuesday.

Russian and Syrian forces have stepped up air raids and shelling on rebel-held areas of northwestern Syria, the heaviest assault in the area since it was declared a demilitarised zone last year.

Villages and towns in northern Hama province and southern Idlib that were targeted overnight on Wednesday are part of a “safe zone”, agreed in September between Russia and Turkey, as part of a deal that averted a major offensive on the area.

Schools and residential areas have been hit, Panos Moumtzis, the UN regional humanitarian coordinator, told Al Jazeera.

“There have been a number of casualties that have taken place in many locations: in health facilities, in schools, in residential areas,” he said, adding about 250,000 people had been displaced

Separately, he told Reuters news agency “the barrel bombing is the worst we have seen in 15 months”.

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A war monitor and activists said on Thursday that four members of a family, including two children, were killed in an air attack in Kansarfa village, in the Idlib countryside.

Since Tuesday, attacks have forced thousands of civilians to flee to camps further north along the Turkish border and damaged four medical facilities, according to civil defence officials and a US medical aid agency working in the area.

“Medical facilities are being evacuated, leaving the most vulnerable with no access to medical care. We are on the edge of a humanitarian catastrophe,” Khaula Sawah, vice president of the US-based Union of Medical Care and Relief Organizations, said in a statement on Wednesday.

Syrian army helicopters have dropped barrel bombs, which are drums or cylinders packed with explosives and shrapnel that cause indiscriminate destruction, killing at least 15 civilians and injuring dozens, rescuers in Idlib said.

Opposition-run civil defence agencies say hundreds of people, mostly civilians, have been killed by Russian and Syrian attacks since the September deal, which prevented a potentially devastating offensive on Idlib and nearby areas held by fighters and now home to more than three million people.

Syrian state media, quoting army sources, blamed rebels for the assaults and said it targeted “terrorist groups” in towns in northern Hama including Kfr Nabouda.

Seven countries to hold Geneva talks

The military escalation is expected to be a focus of talks in Geneva on Friday between envoys from seven countries, including US Ambassador James Jeffrey and UN Special Envoy for Syria Geir Pedersen.

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Idlib is the last major area of Syria still in rebel hands after a string of government offensives backed by Russian air power since 2015 turned the tables in a protracted civil war.

President Bashar al-Assad has regained control over most of the country, with the northeast held by Kurdish groups backed by the United States.

Idlib is held by an array of rebel groups, including the powerful Hay’et Tahrir al-Sham, a coalition of armed groups including those formerly affiliated with al-Qaeda.

Turkey, which has supported the rebels and has troops to monitor the truce, has been negotiating with Moscow to halt the air attacks with little success.

The main Turkish-backed rebel grouping said they were rushing extra fighters to the main fronts to face all “possibilities”.

In response to the escalation, rebels said they mounted several rocket attacks on army positions including the Breideej army base in northern Hama, wounding and killing at least four Russian soldiers in a mortar attack that hit their vehicle.

“We have stepped up our readiness and sent troop reinforcements to confront any attack [by] the regime and the Russians on any area,” said Naji al-Mustafa, spokesman for the National Liberation Front coalition of rebel groups.

“We cannot leave matters like this… We are preparing for any possibility,” he added.