Damascus and its ally Russia have pummelled Idlib province and surroundings over the past month despite a truce deal aimed at staving off a humanitarian catastrophe.

The Britain-based Syrian Observatory for Human Rights monitoring organisation says over 285 civilians have been killed since late April in the enclave, home to almost three million people.

"The pace of air strikes decreased relatively on Thursday compared to previous days," said Observatory head Rami Abdel Rahman.

But bombings still killed seven civilians, including five in the town of Maaret al-Numan on the western edge of the jihadist-held territory, he said.

An AFP photographer reported the strikes hit a residential area, collapsing a building and killing some of those inside.

The body of a victim could be seen still in bed as rescue workers struggled to reach survivors trapped under the rubble.

Idlib province and some surrounding areas are mostly controlled by Hayat Tahrir al-Sham, a group dominated by former members of Al-Qaeda's Syria affiliate.

The United Nations says roughly 270,000 people have been displaced by the fighting since late April and that aid agencies have been forced to halt work in some areas.

Some 30 health facilities and schools have been hit, the UN says.

Despite the surge in attacks, the government has not announced an all-out offensive to retake the entire jihadist enclave.

Russia and rebel supporter Turkey brokered a ceasefire deal in September to avert a government assault it was feared could spark the worst humanitarian disaster of the eight-year war.

The conflict in Syria has killed more than 370,000 people since it started in 2011.