US President Joe Biden has said it is still possible to find survivors in the rubble of a building that collapsed last week near Miami in Florida.

Speaking after meeting families of people who died or are still missing, he said they were going "through hell", and that federal funding for the rescue effort would continue for a month.

He also said it was essential to find out what caused the collapse.

Eighteen bodies have been recovered but 145 people are still unaccounted for.

Rescue efforts resumed on Thursday evening after a 15-hour halt amid concerns about the stability of the remaining structure of the Champlain Towers South. Nobody has been pulled alive from the rubble since the first hours after the disaster.

Miami-Dade County Mayor Daniella Levine Cava said officials were planning for the possible demolition of the remaining structure, and a decision would need to be made "extremely carefully and methodically," given how it would affect the search-and-rescue mission.

Earlier this week, in an interview with news outlet WPLG, a rescue worker recalled hearing a woman's voice underneath the debris - and his team's desperate attempt to get her out.

"We were continuously talking to her... 'Honey we got you. We're going to get to you'," the worker said.

But despite their continuous efforts, the woman died of her injuries.

Addressing relatives in a private meeting, President Biden drew his own experience of grief as he offered his prayers and support. In a video posted on social media, he was seen speaking in sombre tones about the death of his wife and daughter in a car accident.

Later in a news conference, he said the "whole nation" was mourning with the families, and that he was "holding out hope" that survivors could be found.

"They're praying and pleading that 'God, let there be a miracle'," he said. "There is still a possibility that someone could be alive, that someone is breathing."

President Biden also met firefighters and other first responders who have been working at the site and paid respects at the memorial site for the victims alongside First Lady Jill Biden.

Eight days after the disaster, officials are still searching for answers about what might have caused the 40-year-old block's sudden collapse in the town of Surfside, north of Miami.

Experts gathering information at the scene are considering a range of possible causes - from structural defects to environmental influences - and whether a combination of factors may have triggered the fall.

At the news conference, President Biden noted that many of the victims' families mentioned potential causes, such as rising sea levels and climate change. But he added: "I don't think there is at this point any definitive judgement as to why it collapsed."

On Wednesday, it emerged that residents had received a letter in April warning them of worsening structural damage to the building.

Lawsuits are already being filed over the disaster, with the latest accusing the block's building association of "reckless and negligent conduct".

Miami-Dade County State Attorney Katherine Fernandez Rundle said she would convene a special grand jury to examine building safety and "what steps we can take to safeguard our residents" from similar disasters in the future.