Rescuers used their bare hands to dig through the rubble of a five-storey building that collapsed in central Nairobi Monday, killing at least 11 and injuring more than 70.

The building was under construction and there were more than 280 construction workers inside when it collapsed, a worker told The Associated Press.

There are fears that many may be trapped beneath the wreckage, and even though it is night there now, rescuers have set up lights to continue digging for survivors.

Dozens of soldiers, firefighters, police and Red Cross workers rushed to the site to help dig, using crowbars and metal cutters.

"We saw the building sink slowly and then sway from side to side,'' said Serengo Wekesa, who had been working at a neighbouring building.

"As we saw this, we fled the area. We never got that far -- it collapsed."

The total number of dead and injured has not been confirmed yet.

An AP reporter at the scene saw three bodies of those killed in the collapse. Four others reportedly died while being treated, a hospital spokesman said.

More than 70 people were taken to Kenyatta General hospital, said Dr. Samual Ngiru. Six are in critical condition, and the need for blood donations is urgent.

Health Minister Charity Ngilu said she didn't have any information from the private hospitals yet where other victims were taken.

Most of the injuries were a result of the collapse. Some were wounded in the stampede that followed.

A worker who asked not to be identified said an inspector warned last week that the structure was unsafe and they were trying to stabilize the building.

"This is all about building standards," said army Brig. George Kyaka, who was leading the military response. "But those who are alive are the priority now."

"It is very important that we put in place mechanisms to ensure that only properly designed buildings are built," said Vice-President Moody Awori, who was at the scene. He declined to comment further.

At the time of collapse, construction workers were trying to add additional upper floors to the structure.

James Ofunyi, a worker, said they were on their lunch break, and some were asleep in the building when it collapsed.

"I was taking a nap after having had lunch when I heard someone shouting to run, but I didn't get downstairs fast enough because the building started falling," Ofunyi said. "I jumped to the other side."

Two survivors were pulled from the building more than an hour after the collapse, prompting the crowd to erupt in cheers. But there were fears that more people were caught between the floors that collapsed on top of one another.

Parts of the exterior walls were still standing.

With files from The Associated Press