A six-story apartment building collapsed early Wednesday in the capital of Bangladesh, killing at least 11 people and trapping about 25 inside, rescuers said.

The building's 60 residents were asleep when the brick-and-cement structure in Dhaka's impoverished Shakharibazar district (search) crumbled.

Rescuers found seven bodies in the rubble, and four badly injured people died on the way to hospitals, said Selim Newaz, a firefighter at the scene. Two children were among dead.

At least eight injured people, including a 70-year-old woman, had been admitted to a hospital, he said. Some 25 people were believed to be trapped inside.

Newaz said the three lowest apartments of the building were completely collapsed, and most victims were believed trapped there.

"It was a hot and humid night, and I slept on the rooftop of the house," said Tapan Kumar Sen, a survivor. "I was awakened by a big bang to find the top three floors of the building tilted to one side."

Sen said he and five others sleeping on the roof climbed down the water pipes of a neighboring building. They suffered cuts and bruises.

Soldiers used crowbars and hammers to dig through the debris, trying to reach the victims still trapped inside.

The cause of the collapse was not immediately known, but neighbors said the owner recently added three floors to the original three-story building, which was about 200 years old.

"We will investigate the cause of the disaster," Dhaka's Mayor Sadek Hossain Khoka (search) said after visiting the scene.