French rescue workers scrambled Tuesday to find two people still missing in the rubble of a residential building destroyed in a suspected gas explosion, with one man's body already found while a woman and baby were extracted alive.

The woman and baby as well as three others were injured in the blast in the Mediterranean coastal town of Sanary-sur-Mer, which was heard from as far as eight kilometres (five miles) away.

"It was like a bomb blast, a huge deafening noise," said Anita Lonvis, who lives in a nearby street, while her friend, who gave only her first name Habi, said "I thought a plane had crashed."

"It's very likely that the victim is the father of the baby," Houda Vernhet, director of the government's regional authority for the Var region, told AFP.

He was unconscious when located and declared dead after rescue workers spent more than two hours removing him from the unstable wreckage of the three-storey building.

The two people still missing "are a mother, an elderly woman, and her son" who lived on the ground floor of the building, located between two restaurants, Vernhet said.

"For now, we haven't yet found any signs of life from the rubble, but we didn't hear the baby right away, either," said Colonel Eric Grohin, director of the fire service for the Var department.

"We have to be careful, working cautiously to remove the debris and advance step by step toward potential victims," he said.

- 'Not dilapidated' -

Two adjacent buildings were also heavily damaged in the blast that occurred in the port at Sanary, a town of around 15,000 people southeast of Marseille.

The explosion recalled a similar dramatic building collapse in Marseille itself in 2018, when eight people were killed after two buildings collapsed in a working-class district near the city's historic port.

The disaster prompted protests by residents angry that officials had failed to repair the crumbling buildings despite years of complaints.

But the mayor of Sanary-sur-Mer, Daniel Alsters, said the building "was in no way dilapidated," even though "they were old, like all the buildings in the historic centre" of town.

Authorities said rescue workers smelled gas when they arrived at the site.

"The causes aren't known for now. There was a smell of gas, but we can't say anything more while the police inquiry is underway," the regional authorities said in a statement.