SHANGHAI, May 4 -- Two people have been confirmed dead and three others were injured after a gas explosion toppled an old residential building in Shanghai early Sunday morning, local authorities said.

The three-story brick-and-wood structure crumbled at 4:49 a.m. when nine people were in the building in Hongkou District, according to a statement from the district government.

Four people managed to escape while the rest were pulled out by rescuers, said the statement.

Two of the rescued were later confirmed dead by medical staff, it said, adding that three injured survivors undergoing hospital treatment were in stable condition.

An initial investigation showed the accident was caused by a Calor gas explosion, which also damaged a second building.

A woman, surnamed Zhu, who lived in a nearby building, said she was awakened by three huge bangs early in the morning.

"Because it rained at night, I first thought they were thunder," said Zhu. "But I soon realized that it was a building collapse as I found dust was all around, and I immediately called local police," she said.

Some of the buildings on the block are up to 60 years old, and others are make-shift buildings. The buildings, mostly rented by migrant workers, had been scheduled to be demolished for 10 years, said residents.

Local housing authorities said they will check buildings nearby, and if some were found to be in danger of collapse, residents living inside would be relocated.