Rescuers raced against time to find more survivors amid the rubble of a deadly building collapse in South Africa on Wednesday, as search operations were about to enter their third day.

Dozens of people are still unaccounted for almost 48 hours after rescue services started combing through the debris in the southern city of George.

At least seven people were killed as an under-construction five-storey building collapsed on Monday for reasons that are yet to be determined.

Twenty-nine of the 75 workers who were at the site when the incident occurred have been pulled out of the rubble alive, authorities said.

Six have life-threatening injuries, and 16 are in critical condition.

Moses Malala, a foreman who survived the collapse, told AFP he heard a loud sound before the building came crashing down.

Malala, who was working on the roof, said he felt his feet slipping as the building started to fold on one side.

He watched his colleagues fall one by one. Many are still buried under the rubble.

Malala was injured but escaped with his life and has been helping with rescue efforts.

"I have pain too much... I can't sleep" he said. "Since Monday I was here on the site, we try to remove our relatives, our brothers and sisters".

Thirty-nine people remain unaccounted for, authorities said.

The next few hours are critical to save them, they added. The chances of surviving drop dramatically after 72 hours.

More than 200 rescue workers and emergency service personnel divided into three teams searched separate areas on Wednesday.

The building, which collapsed at around 2:00 pm local time (1200 GMT) on Monday, was meant to be a 42-unit apartment block.

On Tuesday night, slight cheers were heard as a survivor was pulled out of the rubble and put onto a stretcher, an AFP correspondent saw.

Another body was retrieved and wrapped in a blanket.

Religious leaders and social workers were at the scene to assist and comfort distraught families.

Men, women and children sang and prayed at the city hall as they awaited news of their loved ones.

One pastor told AFP they were offering "spiritual support" to those affected by the tragedy.