Regime air strikes killed 12 civilians including four at a market on Sunday in a militant bastion in northwest Syria, a war monitor said.

A young girl was among those killed at the market in the town of Maaret Al Numan in Idlib province, the Syrian Observatory for Human Rights said.

Eight other civilians were killed elsewhere by government fire in Idlib, a stronghold of Syria's former Al Qaeda affiliate Hayat Tahrir Al Sham, the Britain-based monitor said.

Idlib is supposed to be protected from a massive government offensive by a September buffer zone deal, but the militant bastion has come under increasing bombardment by the regime and its Russian ally since late April.

An AFP reporter in Maaret Al Numan saw a young man carry the arched body of what appeared to be a young girl out over grey rubble after the air strike.

People walk among debris following a reported air strike by regime forces on Sunday. Abd el-Aziz Qitaz/AFP

Another man retrieved a distressed, dust-covered young girl, slung over his shoulder.

Witness Hamdu Mustafa said he was out shopping when the air strike hit.

Everybody was "in the street selling and buying," he told AFP.

"The planes targeted civilians who were buying food for their children," he said.

Nearby, rescue workers known as the White Helmets directed a bulldozer to clear the debris.

Fighting has raged to the south of the bastion in recent days.

On Sunday, regime forces took back control of the town of Kafr Nabuda in the north of Hama province, the Observatory and state news agency SANA said.

HTS and allied rebels overran part of the town in recent days, after the regime first expelled them on May 8.

The United Nations has warned that an all-out offensive on the Idlib region would lead to a humanitarian catastrophe for its nearly three million residents.

The Observatory says more than 230 civilians have been killed in the spike in violence since the end of April.

More than 200,000 civilians have already been displaced by this upsurge of violence, the United Nations has said.

A total of 20 health facilities have been hit by the escalation — 19 of which remain out of service, according to the UN.

Government steps up air strikes

Syrian government forces pounded positions in the northwest of the country on Sunday, the Syrian Observatory for Human Rights said, in the heaviest day of air strikes since launching a major campaign against the rebel-held territory nearly four weeks ago.

The bombardment helped Russian-backed government forces capture the small town of Kafr Nabouda in northern Hama province, the third time it has changed hands in the latest offensive, sources on both sides said.

The onslaught since late April, focused mostly on southern parts of Idlib province and adjacent parts of Hama and Latakia, marks the most intense conflict between President Bashar Al Assad and his insurgent enemies since last summer.

The bombardment has killed 229 civilians, injured 727 and forced more than 300,000 people to flee since April 28, according to The Union of Medical Care and Relief Organisations (UOSSM), which provides assistance to health facilities.

Rami Abdulrahman, director of the Observatory, said Syrian government planes and helicopters launched more than 280 strikes on Sunday and Russian jets had carried out 15.

Syrian state news agency SANA said Kafr Nabouda had been taken from militants led by a group known as Tahrir Al Sham, previously called the Nusra Front and part of al Qaeda until it broke away and renamed itself.

A spokesman for one of the rebel formations in the area, the Turkey-backed National Liberation Front, confirmed government forces had taken Kafr Naboud after an eight-hour bombardment involving hundreds of rockets and dozens of air strikes.

Agencies