The number of people who have died in the collapse of a high-rise apartment building in Miami has risen to 11.

Another 151 remain missing after part of the Champlain Towers building in the town of Surfside collapsed on Thursday.

The update came during a news conference on Monday afternoon in which it was revealed that experts who investigated the fall of the Twin Towers in the wake of 9/11 have been drafted in to look into the collapse.

US Congresswoman Debbie Wasserman Schultz said experts from the National Institute of Standards and Technology (NIST) are on site gathering preliminary evidence.

She told reporters: "They are the best of the best when it comes to making sure that they do a complete and thorough overview."

However, governor Ron DeSantis cautioned that NIST's work looking into the cause of the collapse will "take a long time".

"It is something that is going to be very thorough and is something that is not going to happen in a day or two. This is going to take a long time," he said.

He said rescue efforts had been hampered by "hazardous" conditions and fires.

"[Rescuers] have been going at it for over 100 hours straight - when there's danger, they run towards it.

"They obviously shepherded a lot of people to safety initially. And they have been every minute of every day since the building collapsed, trying to identify survivors.

"They're putting themselves at risk in order to do that, not only because you could have additional collapse.

"You've had deep seated fires that they had to fight. It's been an incredibly hazardous environment and they've been on the scene non-stop from the very beginning."

Rescue workers digging for a fifth day into the remnants the building stressed that they could still find survivors in the rubble, giving hope to relatives.

Crews were using cranes, dogs and infrared scans as they looked for any signs of life amid the ruins, hoping air pockets may have formed underneath the concrete that could be keeping some people alive.

"We're going to continue and work ceaselessly to exhaust every possible option in our search," Miami-Dade County Mayor Daniella Levine Cava told the news conference. "The search-and-rescue operation continues."

Meanwhile, Andy Alvarez, a deputy incident commander with Miami-Dade Fire Rescue, told ABC's Good Morning America that rescuers have been able to find some voids inside the wreckage, mostly in the basement and the parking garage.

"We have over 80 rescuers at a time that are breaching the walls that collapsed, in a frantic effort to try to rescue those that are still viable and to get to those voids that we typically know exist in these buildings," Mr Alvarez said.

"We have been able to tunnel through the building," Mr Alvarez added. "This is a frantic search to seek that hope, that miracle, to see who we can bring out of this building alive."

A humanitarian delegation from Israel that includes several search-and-rescue experts is also involved in the search.

Israeli Diaspora Affairs Minister Nachman Shai, who is heading up the team, said professionals have told him of cases where survivors were found after 100 hours or more.

"So don't lose hope, that's what I would say," he said.

Nevertheless, some relatives of the missing said they were daunted by the task ahead.

Alfredo Lopez, who lived with his wife in a sixth-floor corner apartment and narrowly escaped, said he finds it hard to believe anyone is alive in the rubble.

"If you saw what I saw: nothingness. And then, you go over there and you see, like, all the rubble. How can somebody survive that?" Lopez told the Associated Press news agency.

Dianne Ohayon, whose parents, Myriam and Arnie Notkin, were in the building, said: "It's hard to go through these long days and we haven't gotten any answers yet."

The building collapsed just days before a deadline for apartment owners to start making steep payments toward more than $9 million in repairs that had been recommended nearly three years earlier, in a report that warned of "major structural damage".

But Ross Prieto, then Surfside's top building official, met residents the following month after reviewing the report when they were assured the building was "in very good shape", according to minutes of the meeting seen by Reuters.

Authorities on Sunday said four people had been recovered from the rubble.

They were identified as Leon Oliwkowicz, 80, and his wife, Christina Beatriz Elvira Oliwkowicz, 74; and Ana Ortiz, 46, and her son Luis Bermudez, 26.

Four of the victims had already been identified: Stacie Fang, 54, Antonio Lozano, 83, Gladys Lozano, 79, and Manuel LaFont, 54.