Russian and Syrian forces intensified air strikes in northwestern Syria killing at least 15 civilians, injured dozens causing indiscriminate destruction in Idlib province.
It is the heaviest assault on the last rebel-held area.
The attacks have forced thousands of civilians to flee to camps further north along the Turkish border.
The targeted villages and towns in northern Hama and southern Idlib were declared a demilitarised zone under a Russian-Turkish deal.
United States has warned violence in the buffer zone will result in the destabilization of the region.
The U.S.-based Union of Medical Care and Relief Organizations vice president Khaula Sawah said, they are on the edge of a humanitarian catastrophe.
She said, medical facilities are being evacuated, leaving the most vulnerable with no access to medical care.
Civil defense agencies say mostly civilians have been killed.
Syrian state media blamed rebels for the attacks, saying terrorist groups and Al Qaeda-inspired groups were targeted.