Rescuers said Tuesday they had made contact with 11 people as they raced to save dozens still unaccounted for nearly 24 hours after the deadly collapse of a building in the South African city of George.

Twenty-seven people have been pulled out of the debris of the five-storey building, which collapsed on Monday while under construction, municipal authorities said.

Six of those have died, they said in George, which lies 400 kilometres (250 miles) east of the city of Cape Town.

Contact has been made with 11 people who are under rubble among 48 still unaccounted for, they said.

"We are in contact with 11 people" trapped under rubble, Colin Deiner, head of rescue operations, told a news conference in Cape Town.

The main focus is to get everyone out, he said.

"Once we have got all the people that we know about out, we are going to start a process called delayering," Deiner said.

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

Provincial premier Alan Winde told the same news conference that the death toll had risen from five to six.

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

"Three teams of rescue personnel are currently working on three different areas within the site of the collapsed building," they said.

The reasons for the collapse of the building, which included an underground parking garage, are still unknown.

Construction companies and city authorities have been working to draw up a list of those still unaccounted for, according to the city.

President Cyril Ramaphosa offered his "deep condolences to the relatives and friends" of the victims 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 continued throughout Monday night with strong floodlights illuminating the cordoned-off site.

A coordination post was set up to run the operations, which involve numerous emergency services and over 110 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.

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

But officials urged onlookers and members of the public to stay away from the "dangerous" site.

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

"No stone will be left unturned in making sure that we get to what caused this accident," Winde vowed.