Rescue efforts were underway after three people were killed and dozens remained trapped after a multi-storey building under construction collapsed in the South African city of George, authorities said.

Twenty-four people were rescued from the site and sent to hospitals, the municipality of George, a coastal city east of Cape Town, said in a statement on Tuesday.

Fifty-one people remained trapped, according to a statement from the municipality of George.

Seventy-five members of construction crew have been confirmed on site at that time of the incident, Western Cape premier Alan Winde said earlier.

CCTV footage obtained by Reuters showed a cloud of dust as the building crumbled on Monday afternoon.

I saw one guy was working and then ˜boom and I saw the whole building collapsed¦ Im also traumatised. It is very sad, Theresa Jeyi, a local councillor, told reporters at the scene.

The provincial government said police and rescue teams with sniffer dogs were on site.

Rescue workers on site were trying to recall where they heard calls from people, George Mayor Leon van Wyk told SABC, while rescue equipment was on its way.

This is going to be a really lengthy exercise and probably going to go right through the night, he said. (CNN)