Relatives of 44 construction workers trapped under a collapsed building in the South African city of George faced a fourth day of anguished waiting on Thursday as heavy machinery worked at the site in a race against time to find any survivors.

Of 81 people who were on site when the five-storey private residential building collapsed on Monday, eight have been confirmed dead and 29 alive, 16 of them in a critical condition, according to the latest figures from George municipality.

"A number of families... (are) waiting, not knowing what has happened with their children, with their husbands, their uncles," Thulas Nxesi, South Africa's employment and labour minister, told reporters on site.

Nxesi said the government would conduct its own investigation. The cause of the collapse is yet to be established.

"We still hope and pray that we can get people out alive," Anton Bredell, a Western Cape provincial minister, said.

The identities of the missing have not been made public, but a list of names was circulating among groups of relatives who have congregated at the site since Monday, desperate for news of their loved ones, state broadcaster SABC reported.

An earth mover could be seen removing broken slabs of concrete from the collapsed building, now a chaotic pile of masonry and twisted steel reinforcements. Sniffer dogs had been brought in to search the site.

The incident has prompted an outpouring of solidarity in George, with local companies providing equipment and volunteers setting up coffee stalls for waiting relatives and rescue workers.

Nearly 700 people were involved in rescue efforts on Wednesday.

Rescue teams had been hearing trapped survivors, a disaster management official said on Tuesday, but there have been no further updates about that since then.