The construction workers had just finished lunch and many were taking a nap when the five-story building began to sway, then quickly collapsed, killing at least 11 and injuring dozens, survivors and officials said.

Rescuers dug frantically through the rubble with bare hands, metal-cutters and crowbars to reach survivors who shouted through drainage pipes and holes, while lights were set up so the search could continue into the night.

One person called a relative on a cell phone and said he was alive under the concrete and steel. A trapped person's hand could be seen waving for help from beneath a beam.

More than 280 construction workers were inside when the unfinished structure in central Nairobi came down, but officials said it was unclear how many were still caught in the rubble.

"Some were lying down for a nap. They were too tired after working for at least six hours," said food vendor Jane Wanji, 32. "We felt the building shake and those of us who were on the rooftop jumped onto the roof of the next building. We then watched it collapse."

Adding to the chaos, a bus crashed through the medical triage area, injuring a medic and slamming into a parked ambulance. Its brakes had apparently failed.

Wanji was one of several women who had set up stalls inside the building to sell food to workers and some had brought their children with them. A 2-year-old boy was among the injured.

Immediately after the collapse, hundreds of people formed lines to carry away chunks of concrete and wooden scaffolding, which a front-loader then carted away. Dozens of soldiers, firefighters, police officers and Kenya Red Cross workers worked to free those trapped.

Rescue workers were running out of oxygen to administer to survivors as they were being pulled out. There also was a shortage of power saws and other heavy equipment, Joshua Toro, assistant minister for roads and public works, said after talks with emergency workers.

Israel's army was sending a search and rescue team including doctors and 45 professional searchers along with humanitarian and rescue supplies, the army said.

The U.S. and British governments also have offered to send experts to help in the search and rescue operation, but it was not clear when the rescue workers will arrive, said Abbas Gullet, the secretary-general of the Kenya Red Cross.

At least 90 people were pulled out of the rubble alive, but six of those later died of their injuries, police spokesman Superintendent Jaspher Ombati said at the scene.

Dr. Gilbert Oburu, who was coordinating emergency treatment for survivors, said he saw five dead bodies at the site.

More than 70 injured were taken to Kenyatta General Hospital, said Dr. Samuel Ngiru, who works in the trauma department. "If we do not get more blood, people will die," he said.

Most of casualties were from the building collapse. But some were injured in a stampede that followed.

"I was coming home from school when I heard a whoosh and people started running, and I was pushed over," Evans Omolo, 11, said while clutching his broken arm at the hospital. "I couldn't see what was happening, but people were jumping on me as they ran past."

Tens of thousands of people ran to the site, clogging roads and climbing on top of the rubble to watch the rescue. Riot police deployed to the area and beat back the crowds with truncheons, clearing roads for emergency vehicles. Most of the onlookers were gone by nightfall.

Two survivors were pulled out more than an hour after the collapse, and the crowd erupted in cheers. But there were fears that others were caught between the floors that collapsed on top of one another.

One construction worker, who would not give his name, said an inspector had warned last week that the structure was not safe and they were trying to stabilize the building.

"This is all about building standards," said Kenyan 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," Vice President Moody Awori visited the scene to check on the rescue effort.

President Mwai Kibaki announced he was cutting short an official visit to Sudan to coordinate rescue efforts.