Rescuers were using cranes, drills and their bare hands to try to reach dozens of people trapped when a multi-storey building being built in the South African city of George collapsed, killing at least six people.

Of the 75 workers who had been on the construction site, 48 remain unaccounted for on Tuesday. Authorities have made no comment about what caused it to collapse on Monday.

While the rescue teams are able to communicate with 11 people buried in the wreckage, families gathered waiting for news of their loved ones were in tears, fearing the worst.

"We treat everybody as still alive," Colin Deiner, chief of disaster management for the Western Cape province, told a press conference.

There have been moments of hope. People clapped and cheered as rescue workers pulled a person out alive from among broken concrete slabs and twisted steel reinforcements. They have made contact with other survivors as they scour the site with sniffer dogs.

"We have one area where four people are in a basement and we've been communicating with them, so that's quite a big operation that's taking most of the day to get them out," Deiner said.

So far 27 people have been retrieved from the site, in the coastal city east of Cape Town, including at least six dead.

President Cyril Ramaphosa has called for an investigation.

Liatel Developments, the contracted builder of the structure, said it was trying to assist those on site of the five-storey residential building.

"The investigations to follow obviously will reveal what has transpired and what has happened, but at this point in time it's just saving as many people as we possibly can," company director Theuns Kruger told Reuters.

CCTV footage seen by Reuters showed a cloud of dust as the building crumbled.

A local councillor told reporters on Monday she had heard a "boom" and then the building collapsed.