NAIROBI, Oct. 21 (Xinhua) -- Kenyan rescuers have retrieved 11 bodies as they are stepping up the search for about 20 builders trapped after a building collapsed on the outskirts of Nairobi over the weekend.

Kenya Red Cross Society (KRCS) Public Relations Manager Titus Munguo confirmed on Wednesday that three more bodies were retrieved on Tuesday night, bringing the number pulled from the wreckage in Kiambu town to 11.

"The rescuers retrieved three more bodies last night from the multi-storey building which was under construction in Kiambu town. They have retrieved 11 bodies so far," Monguo told Xinhua by telephone.

The Red Cross official said more people are still trapped under the building and the rescue efforts are expected to go on throughout the day.

"The rescue teams continue digging into the rubble in search of people who were communicating to others outside. We expect to zero in where communications was coming from and expect to call the rescue efforts by the end of the day," said Mongou.

It is not clear what caused the building collapse in Kiambu, about 25 km northwest of the Kenyan capital Nairobi.

Monguo said some of the eyewitnesses reported hearing a loud bang when the building cracked before collapsing.

However, the Kenya Architectural Association has accused building companies of flouting safety regulations.

The architecture association had warned in a report released last week that about 65 percent of the country's buildings do not meet the required standards.

Mungou said counseling centers have been set up near the collapsed building to assist the survivors and the relatives of those still trapped.

"We have tents there. One of them is for tracing of missing persons. Until yesterday night 30 names had been presented by families as missing people and we now that this can also change," he explained.

Relatives of people feared trapped have gone to the site hoping to see their loved ones pulled out alive.