Officials in the city of George, on the country's southern coast, say 49 people are still trapped while 21 have been taken to various hospitals, at least 11 of them having suffered severe injuries.

More than 100 emergency staff and other responders have worked through the night using sniffer dogs and tall spotlights to try to locate workers buried for more than 12 hours.

It's feared some are under huge slabs of concrete that fell on them when the five-storey building fell on Monday afternoon, local time, in the city, around 250 miles (400 km) east of Cape Town.

Large cranes and other heavy lifting equipment have been brought in to remove debris.

Western Cape premier Alan Winde, the head of the provincial government, who was at the scene, said: "Seventy-five members of construction crew have been confirmed on site at that time of the incident.

"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."

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

CCTV footage recorded at 2.09pm obtained by Reuters showed a cloud of dust over the neighbourhood as the building's concrete structure and the metal scaffolding around it came crashing down.

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

Pictures from a drone camera showed a completely flattened construction site, with parts of the building's roof lying on top of the rubble.

As family and friends of the workers gathered at nearby municipal offices, George's executive mayor Leon Van Wyk said: "Our thoughts are with the families and all those affected who continue to wait on word of their loved ones."

Read more:
Israel rejects ceasefire proposal
China 'hacked MoD'
The Met Gala - as it happened

Theresa Jeyi, a local councillor, told reporters at the scene: "I saw one guy was working and then 'boom' and I saw the whole building collapsed... I'm also traumatised. It is very sad."

Authorities are investigating what caused the disaster and police have opened an investigation, but there was no immediate information on why the building suddenly collapsed.