Rescue workers reported on Tuesday making contact with 11 people under the rubble in a race to save dozens still unaccounted for after a building collapsed in the South African city of George killing at least seven.

Thirty-three people have been pulled out of the debris of the five-storey apartment block, which caved in Monday afternoon while under construction, municipal authorities said.

Seven of those have died, the authorities said in George, 400 kilometres (250 miles) east of Cape Town.

Forty-two people were still unaccounted for, including the 11 with whom contact had been established, the authorities said.

With the second night of rescue work underway, emergency services personnel surrounded a hole where a survivor was pulled out of the rubble and put onto a stretcher, an AFP correspondent saw.

As they emerged, slight cheers of relief were heard.

Another body was retrieved and wrapped in a blanket.

Crowds of onlookers stood behind the barricade tape.

"We are in contact with 11 people" trapped under rubble, Colin Deiner, head of rescue operations, had told reporters.

Life had been detected under three areas of the site.

- 17 hours work to rescue one survivor -

On the sidelines of the site, Deiner told AFP it was a "long, involved operation with so many complexities" and an "extensive search with dogs, with systemic equipment" required.

He added it took rescue personnel 17 hours to bring one of the victims out of the rubble after they were initially located.

Recalling that the international standard for such operations is three days, George mayor Leon van Wyk told a local television station in the early evening that "over the next 40-odd hours we are hoping to rescue as many people" as operations entered a second day.

The main focus is to get everyone out, Deiner said. "Once we have got all the people that we know about out, we are going to start a process called delayering," he added.

"We would then start lifting the different floors off each other," he said, adding it would be "tricky" to remove the concrete trapping the building workers.

A construction crew of 75 people was on site at the time of the collapse at around 2:00 pm local time (1200 GMT) on Monday, authorities said.

Three rescue teams were working on three different areas of the collapsed building, which included an underground car park.

The reasons for the collapse were still unknown.

The construction plans for a 42-unit apartment block had been approved by the city in July.

President Cyril Ramaphosa offered his "deep condolences to the relatives and friends" and said his thoughts were with the families of the dead and missing.

He called for an investigation, saying it must "bring closure to the community and prevent a repeat of this disaster".

George, which has a population of about 160,000, is a picturesque coastal city located on the tourist trail along South Africa's southern coastline.

- Diggers, sniffer dogs -

Photos shared by the city council showed a flattened construction site with emergency services on the ground.

The broken roof of the building was still clearly visible atop the pile of rubble.

Rescue operations had continued throughout Monday night with floodlights illuminating the cordoned-off site.

A coordination post was set up to run the operations, which involve numerous emergency services and over 200 personnel.

More diggers and sniffer dogs were dispatched from Cape Town, emergency services said.

Officials said relatives of those trapped had been asked to gather at the city hall near the construction site, where they would be taken care of.

Religious leader and social workers were present to assist and comfort emotional families.

Men, women and children sang and prayed at the city hall as they awaited news on their loved ones, an AFP correspondent saw.

One pastor told AFP they were offering "spiritual support" to those affected by a tragedy.

"Our thoughts are with the families and all those affected, who continue to wait for word of their loved ones," van Wyk said.

George is run by the Democratic Alliance, the leading opposition party, which also controls the Western Cape province.

© Agence France-Presse