Rescuers on Monday worked to free the body of a pregnant nurse from the rubble of collapsed buildings in Sicily, after a massive explosion which killed at least seven people.

The blast, likely caused by a gas leak, tore through four residential buildings late Saturday in the southern town of Ravanusa, with one survivor describing it "as if a bomb had gone off".

The search "continues unabated" for two more people missing after the disaster, firefighters said.

A photograph on the firefighters' Twitter account showed them standing amid the rubble, as "a fresh day of searching painfully begins".

The blast levelled four structures, including a four-storey apartment building, in the central residential district of the town of nearly 11,000 inhabitants, according to Italy's civil protection agency. 

Images from the scene showed a mass of concrete rubble, wooden beams and mangled steel in a large empty space, with neighbouring buildings charred and damaged.

The victims included nurse Selene Pascariello, 30, who was nine months pregnant, and who had been due to give birth next week, according to the Corriere della Sera newspaper.

Pascariello's body was found alongside that of her husband Giuseppe Carmina, and his parents. The couple had been visiting the soon-to-be grandparents on the third floor of their building.

The family had known well one of the first victims to be found, retired high-school teacher Pietro Carmina, who had recently recovered from a life-threatening case of coronavirus, it said.

A tremendous roar

Two women were recovered alive from the debris early on Sunday after being found by sniffer dogs, but rescuers have not heard further signs of life.

An investigation has been opened into the cause of the explosion, which authorities said was most probably a gas leak.

Local resident Calogero Bonanno said "neighbours had told me there was a smell of gas".

"I heard a tremendous roar, as if a bomb had gone off or a plane had crashed into the house," he was cited as saying by Italian media.

"Then the window frames exploded. We immediately went down to the street, there was fire everywhere, rubble all around," he said after fleeing along with his wife, three children and in-laws.

"It's a miracle we're alive".

Natural gas distributor Italgas said in a statement it had received no reports of gas leaks in the week leading up to the incident.

No construction work was under way in the section of pipeline affected in the blast and the town's distribution network was fully inspected in both 2020 and 2021, it said.

The Repubblica daily said the town's gas pipelines -- installed 36 years ago -- were among the oldest in Italy, and ran through unstable ground, susceptible to soil erosion and landslides.

Sicily, one of Italy's poorest regions, suffers from sub-standard and ageing infrastructure.

Many homes and other structures constructed in past decades were built using cheap, sub-par materials that make them more prone to collapse, often because of interference in building contracts by the Mafia.