Rescue teams trying to find dozens of construction workers missing since a multi-story apartment complex collapsed in South Africa have made contact with 11 people buried alive beneath the wreckage, authorities said on Tuesday.

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

That enabled emergency responders to locate the man, although he was still trapped and had not 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 coastal 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,” Mr Deiner said at a press conference. “It is very, very difficult if you are working with concrete breakers and drillers close to people.”

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

“Our big concern is entrapment for many hours, when a person’s body parts are compressed,” Mr Deiner said. “So you need to get medical help to them. We got our medics in as soon as we possibly could.”

Mr 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.

More than 100 emergency personnel and other responders were on the scene, using sniffer dogs to try to locate the 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.

There were 75 workers on the construction site when the building collapsed, the George municipality said.

It said three teams of rescuers were working at separate sites around the collapsed building where they believed construction workers were likely to be.

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 investigating what caused the tragedy and a 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.

Marco Ferreira, a local representative of the Gift of the Givers nongovernmental organisation, was at the site with a team to offer support and food and drink to the rescuers on Monday.

Gift of the Givers is a charity that often helps during disasters in South Africa.

It also provided three sniffer dogs and handlers to help with the search, Mr Ferreira said.

“The situation at this stage is still very much in the rescue stages,” Mr Ferreira told the eNCA TV news channel.

“We don’t know, it’s probably going to carry on for days.

“There are some cranes there to help lift some concrete.

“But it’s not a pretty sight.”

The provincial Western Cape government sent the head of its disaster response unit from Cape Town to George to oversee the rescue operation and Western Cape premier Alan Winde, the head of the provincial government, was also at the scene.

Mr Winde said the provincial government had also sent extra resources to assist.

“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,” he said in a statement.

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