Syrian President Bashar al-Assad attends an interview with a Greek newspaper in Damascus, in this handout released May 10, 2018. Photo: SANA / Handout via Reuters / File.

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.

Air and ground strikes killed 12 people in several areas including the town of Maarat al-Numan, the Observatory 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 Organizations (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.