Nearly half of 3 million inhabitants of northwestern Syria, a rebel stronghold, have fled due to regime bombardment.

At least 25 people have been killed in aerial bombardment carried out by Russian jet fighters in Idlib region, with schools and medical centres knocked down during a continued Syrian military offensive.

The rebel stronghold of northwestern Syria despite being part of a buffer-zone deal has come under deadly regime bombardment in recent weeks, sparking fears for its roughly three million residents.

At least 13 people were killed, including women and children, after Russian Sukhoi jets dropped bombs on the village of Jabala in southern Idlib province, residents and civil rescuers told Reuters news agency.

Russian jets were also behind several raids that hit the town of Khan Sheikhoun, Kafr Battikh and several other villages that left at least another 12 civilians dead, according to another local rescuer.

Rescuers say the major aerial campaign that Moscow has thrown its weight behind since it was launched in earnest at the end of April has killed over 1,500 people with more than half of the death toll civilians.

More than 300,000 people have fled the front lines to the safety of areas near the border with Turkey, UN and aid agencies said.

The Russian-backed offensive has so far failed to make major inroads into rebel territory in northern Hama and southern Idlib provinces, where mainstream rebels backed by Turkey alongside fighters aligned to al-Qaeda-linked groups are putting up fierce resistance in their last remaining bastion in Syria.

Russia and the Syrian army deny allegations of indiscriminate bombing of civilian areas or a campaign to paralyse everyday life in opposition-held areas and say they are fighting al-Qaeda-inspired fighters.

Moscow blames the rebels for breaking a truce by hitting government-held areas and said Turkey has failed to live up to its obligations under a deal brokered last year that created a buffer zone in the area that obliges it to push out rebels.

Nearly half of estimated three million inhabitants in northwest Syria – including Idlib province and parts of neighbouring provinces – had already fled fighting elsewhere, according to the United Nations.

Civilians in rebel-held areas, where many oppose returning to Syrian President Bashar al-Assad’s one-party rule, look to Turkey which has steadily built up a military presence in the area as a protector against the Russian-led attacks.