BEIRUT (Reuters) - The Syrian has government captured the town of Qalaat al-Madiq in northwest Syria, some of its residents and a war monitor said on Thursday, as it pushes into the biggest remaining rebel territory under a massive bombardment.

Syria’s army, backed by Russian air power, launched ground operations this week against the southern flank of the rebel zone consisting of Idlib and parts of adjacent provinces.

The area is nominally protected by a Russian-Turkish deal agreed last year to avert a major new battle.

Qalaat al-Madiq was the rebel area closest to the Russian Hmeimim airbase at Latakia, which insurgents have previously targeted with rocketfire.

It was also the entrance point into rebel territory for many insurgents and civilians who were evacuated from territory captured by the army under surrender deals negotiated with the government over recent years.

Local residents said Syrian government forces had captured Qalaat al-Madiq and two nearby villages - Tal Hawash and al-Karkat.

The Britain-based Syrian Observatory for Human Rights war monitoring group said rebels had withdrawn there after being nearly encircled by the army.

The fighting has pushed 150,000 civilians from their homes, raising concerns of a new humanitarian crisis in northwest Syria. Some 13 health facilities have been hit in the bombing, the U.S.-based Union of Medical Care and Relief Organizations which funds some hospitals in the area said on Wednesday.