An Australian couple are feared missing in the devastating Miami building collapse that has already claimed at least one life.

Rescue crews are combing tonnes of rubble for anyone who may have survived the late-night collapse of part of the oceanfront residential tower.

Miami-Dade Police Director Freddy Ramirez said 99 people remained unaccounted for.

More than 50 others whose whereabouts were initially unknown had since been located.

“Fire and rescue are in there with their search team, with their dogs. It’s a very dangerous site right now. Very unstable,” Director Ramirez said.

“They’re in search-and-rescue mode, and they will be in that mode for a while. They are not quitting. They’re going to work through the night. They are not stopping.”

Australian Joseph Waks, who has lived in Miami for 12 years, told the ABC he feared other Australians might have been in the building when the disaster occurred about 1.30am Thursday (local time).

“It looks like somebody came and bombed our building … We are all mourning here together,” Mr Waks said.

“We have friends who live in that building from Australia and they are unable to be communicated with.”

He later said his missing friends were a couple aged in their 70s or 80s who were originally from Sydney. They spend part of every year in Florida.

“It is devastating … They both became grandparents yet again a few hours before the tragedy.”

“They have been in and out of this community for many, many years. Back and forth from Sydney and Melbourne to Miami.”

The Australian Embassy has not confirmed whether Australians are involved.

But Florida Senator Marco Rubio said many of the people feared missing were foreigners.

Tweet from @marcorubio

Mr Waks also said that family members have been asked to give DNA, but have been given little information about survivors.

“Most of the family members are behind me now in one of the rooms over there giving DNA, so I don’t know if that is positive news, but I don’t think so,” he said.

Emergency responders and officials were still looking for people who might be in the rubble, as well as trying to identify residents who were not home at 1.30am on Thursday when an entire side of the 12-storey building gave way and fell to the ground.

Barry Cohen, 63, said he and his wife were asleep in the building when he heard what he thought was lightning.

The couple went onto their balcony, then opened the door to the building’s hallway to find “a pile of rubble and dust and smoke billowing around”.

“I couldn’t walk out past my doorway,” Mr Cohen said. “A gaping hole of rubble.”

Director Ramirez said the numbers of known casualties and people missing were likely to fluctuate.

“I don’t want to set false expectations,” he said.

“This is a very tragic situation for those families and for the community.”

A fire official said 35 people were rescued from the building in Surfside, a seaside enclave of 5700 residents on a barrier island across Biscayne Bay from the city of Miami. They included two pulled from the rubble as teams used dogs and drones in the search for survivors.

Officials said the building, built in 1981, was having repairs and another building was being built next door, although the cause of the collapse remained unclear.

“We have 51 people that were assumed to have been there, but you don’t know between vacations or anything else, so we’re still waiting,” Miami-Dade county commissioner Sally Heyman told CNN.

“The hope is still there, but it’s waning.”

The search effort was slowed by at least one fire that burned at the site as emergency crews doused the rubble with water, local media reported.

The Champlain Towers South had more than 130 units, about 80 of which were occupied.

Tweet from @MiamiDadeFire

The tower had a mix of seasonal and year-round residents. Surfside Mayor Charles Burkett said while a guest log was kept, it was not always known when owners were in residence.

Included in the missing are 22 South Americans – nine from Argentina, six from Paraguay, four from Venezuela and three from Uruguay, according to officials in those countries.

Among the Paraguayans are Sophia López Moreira, the sister of first lady Silvana Abdo, her husband Luis Pettengill, her three children and the family’s assistant, the country’s authorities said.

Mr Burkett said building manager had told him the tower was quite full when it collapsed, and the death toll was likely to rise.

“The building is literally pancaked,” he said.

-with agencies