Russian and Syrian forces intensified air strikes and ground shelling in northwestern Syria overnight in one of the heaviest assaults on the last rebel bastion in recent months, opposition members, rescuers and residents said on Thursday.

Axar.az reports citing Reuters that, the targeted villages and towns in northern Hama and southern Idlib fall within a demilitarized zone agreed last September between Russia and Turkey. Earlier this week Washington warned violence in the buffer zone “will result in the destabilization of the region”.

Since Tuesday the Russian and Syrian military attacks have forced thousands of civilians to flee to camps further north along the Turkish border and damaged four medical facilities, according to Idlib civil defense officials and a U.S. medical aid organization 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 U.S.-based Union of Medical Care and Relief Organizations -USA (UOSSM US) 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 injured dozens, rescuers in Idlib province said.

Opposition-run civil defense agencies say hundreds of people, mostly civilians, have been killed by Russian and Syrian strikes since the September deal which prevented a potentially devastating offensive on Idlib and nearby areas controlled by rebels and now home to over 3 million people.

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

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

A military defector told Reuters on condition of anonymity that the latest campaign could signal a ground offensive aimed at taking control of territory in northern rural Hama province and southern Idlib countryside.