AMMAN (Reuters) - The Syrian army, aided by Russian warplanes, attacked rebel-held towns in northwestern Syria on Wednesday in the most extensive bombardment in months against the last remaining rebel bastion in the country, rebels, rescuers and residents said.

Rebels who have fought to topple President Bashar al-Assad for eight years are now largely confined to the enclave in the northwest near the Turkish border. Around four million people now live there, including hundreds of thousands of opponents of Assad who fled there from other parts of the country.

The enclave is protected by a “de-escalation zone” agreement brokered last year by Assad’s main international backers Russia and Iran, and Turkey which has supported the rebels in the past and has sent troops to monitor the truce.

Residents said Russian planes conducted at least 12 aerial strikes on residential areas in Idlib city, including a civilian prison on its outskirts, where they said dozens of prisoners escaped. At least 10 civilians were killed and forty five injured.

Russia’s defense ministry confirmed it had hit Idlib in coordination with Turkey, targeting drones and weapons stores of the Hayat Tahrir al-Sham (HTS) jihadists it said were intended for use in an attack on a major Russian air base near the Mediterranean coast.

The jihadist group vowed to take revenge, saying they had recaptured most of the prisoners who escaped whom it said included members of a cell working for Russian intelligence it claimed were behind bombings in Idlib city last month that killed 17 civilians and injured dozens.

“There is no solution against the Russian occupier, Assad’s forces and the Iranian militias except confrontation and confrontation only,” Hayat Tahir al Sham said in a statement.

The Syrian army has escalated its shelling of the enclave since early February. The attacks have killed dozens of civilians and injured hundreds, and led to tens of thousands of people fleeing frontline areas to camps and towns closer to the Turkish border, rescuers and aid agencies said.

The Syrian army denies targeting civilians and says the army is responding to stepped-up attacks staged by al Qaeda-inspired fighters who aim to wreck the truce and control the area.

Residents along the border area with Turkey could hear heavy overnight aerial strikes that covered a wide stretch of territory from rebel-held areas near government-held Latakia province on the Mediterranean to Idlib city toward the east and extending to adjoining opposition-held parts of northern Hama.

“They burnt the land ... The sounds were heard very clearly,” said Ibrahim al Sheikh, a father of five in the border town of Atmeh. He quoted relatives as saying the shelling was the heaviest yet in the two weeks of escalation.

The escalation in the northwest is taking place as a U.S.-backed Kurdish-led militia has launched a separate assault on the final bastion of Islamic State fighters on the opposite, eastern end of the country, creating turning points on both major fronts of Syria’s multi-sided civil war.

‘WHOEVER DID THIS IS A BEAST’

In the northwest, residents said white phosphorous munitions were fired overnight on the town of al Tamana in northwestern Idlib countryside, where rescue workers on Wednesday said they put out several fires caused by more than 80 rocket strikes.

Among the targets of the aerial campaign was a makeshift tent camp in Kfr Amim, east of Idlib city, that shelters displaced families, where two women were killed and at least 10 children injured when bombs landed after midnight.

“Whoever did this is a beast, truly a beast. It’s a camp with only women and children. There is nothing we can say except that this Russian beast is coming to kill,” said Laith al Abdullah, a civil defense worker in Sarqeb town who helped in the rescue effort, reached by mobile phone.

Rocket shelling from a major army base in Joreen, in Hama province, escalated a week-long bombardment of rural areas near the town of Jisr al Shaqour, said Ahmed Abdul Salam, a rebel commander in the Turkey-backed National Liberation Front.

A Russian army base, south of government-controlled Halafaya town, also targeted Kafr Zeita in northern Hama countryside while cluster bombs hit several rebel-held towns in southern Idlib, rebels said.

The stepped-up bombardment has depopulated opposition-held towns in the buffer zone that straddles parts of Idlib to northern Hama and parts of Latakia province.

The opposition-held city of Khan Sheikhon had become a ghost city with most of its more than 70,000 people fleeing, said Yousef al Idlibi, a former resident who moved to Idlib city.

Turkey, which began patrols in the buffer zone on Friday, has condemned what it said were increasing provocations to wreck the truce, and warned that a bombing campaign by the Russians and the Syrian army would cause a major humanitarian crisis.

Many residents are exasperated by the failure of Turkish forces to respond to the bombardments. The Syrian army has called for Turkish forces to withdraw.