Rescue teams trying to find dozens of construction workers missing since a multistorey apartment complex collapsed in a coastal city in South Africa have made contact with 11 people buried alive beneath the mangled wreckage, authorities say.

One man called his wife from underneath the rubble of the five-story building that had been under construction when it collapsed, the head of the rescue operation said.

That enabled emergency responders to locate the man, although he was still trapped and hadn't yet been brought out.

Six workers have been confirmed dead and there are fears that the death count could rise sharply. There is no news on 37 other people unaccounted for amid the huge slabs of concrete and metal scaffolding that came crashing down when the building collapsed in the city of George, about 400km (250 miles) east of Cape Town.

Twenty-one other workers were rescued from the site and taken to various hospitals, with at least 11 of them suffering severe injuries, the George municipality said.

Colin Deiner, head of the provincial Western Cape disaster management services, said the search-and-rescue operation would likely take at least three days.

“We are going to give it the absolute maximum time to see how many people we can rescue,” Deiner said.

“It is very, very difficult if you are working with concrete breakers and drillers close to people.”

Deiner said it would take most of Tuesday to extract the 11 workers that rescue teams were in contact with, some of whom had limbs trapped under concrete and couldn't move. Four of the workers are in what was the basement of the building, Deiner said.

“Our big concern is entrapment for many hours, when a person’s body parts are compressed.,” Deiner said.

“So, you need to get medical help to them. We got our medics in as soon as we possibly could.”

Deiner said it was possible that there were more survivors deeper in the wreckage and a process of removing layers of concrete would begin after the 11 located workers were taken out.

Rescue efforts under way after deadly building collapse. – Reuters

More than 100 emergency services and other personnel worked through the night, using sniffer dogs to try to locate workers. 

Large cranes and other heavy lifting equipment were brought to the site to help with the rescue effort and tall spotlights were erected to allow search and rescue personnel to work through the night.

Deiner said a critical part of the rescue operation came when they ordered everyone to remain quiet and shut off machinery so they could listen for any survivors. That's when they located the 11 workers, he said.

“We were actually hearing people through the rubble,” Deiner said.

Several local hospitals were making space in their trauma units in anticipation that more construction workers might be brought out alive. More than 50 emergency responders had also been brought in overnight from other towns and cities to help, including a specialized team that deals with rescue operations in collapsed structures.

Family and friends of the workers gathered at the nearby municipal offices.

“Our thoughts are with the families and all those affected who continue to wait on word of their loved ones,” George Executive Mayor Leon Van Wyk said.

Authorities were starting investigations into what caused the tragedy, and a criminal case was opened by police, but there was no immediate information on why the building suddenly collapsed. 

CCTV footage from a nearby home showed the concrete structure and metal scaffolding around it come crashing down at 2.09pm on Monday afternoon, causing a plume of dust to rise over the neighbourhood.

People came streaming out of other buildings after the collapse, with some of them screaming and shouting.

Rescuers dig through rubble after building collapse in South Africa. – Reuters

Alan Winde, the Premier of the Western Cape province, said there would be investigations by both the provincial government and the police.

Authorities declined to give out any information on the construction company involved but said that under city law the private company's engineers were responsible for the safety of the building site until its completion, when it would be handed over to the city to check and clear.

Winde said the priority was the rescue effort and investigations would unfold after that.

“All the necessary support has been offered to emergency personnel to expedite their response. At the moment, officials are focused on saving lives. This is our top priority at this stage,” Winde said.

The national government was being briefed on the rescue operation, he said.

South African President Cyril Ramaphosa released a statement offering his condolences to families of the victims and also called for investigations into the cause of the collapse.