image copyrightReutersimage captionRescue teams continue to scour the scene of the partial building collapse for survivors

Residents of a Florida apartment building that collapsed last week received a letter in April warning of worsening structural damage.

The letter, obtained and reported by US media, was sent by the president of the apartment block's building association.

It said damage that was first reported in 2018 had "gotten significantly worse" and would soon "begin to multiply" without immediate repairs.

At least 11 people were killed and 150 more are still missing in the collapse.

The letter is the latest in a string of documents that have emerged providing evidence that the Champlain Towers South building in Surfside, north of Miami, had known structural issues.

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

  • What could have caused building collapse?
  • Why is the rescue effort taking so long?
  • Relatives face agonising wait for news

The April letter, sent by the president of the Champlain Towers South Condominium Association, said that "observable damage" had gotten "significantly worse" and warned that "concrete deterioration is accelerating".

The letter also warned about roof damage, and estimated that fixing the building's problems would cost about $15m (£10.8m)

It follows reports about findings from a 2018 inspection that urged timely repairs after "major structural damage" was found, including "abundant cracking" in an underground parking garage.

media captionFirefighters search for survivors under collapsed Miami building

Questions over the collapse are mounting at the same time as a desperate search operation continues.

Dozens remain missing in the rubble of the 12-floor apartment building, which partially collapsed in the early hours of Thursday as scores of residents slept.

Officials insisted on Tuesday that the operation, now in its sixth day, remains a search and rescue one. "Nobody is giving up hope here. Nobody is stopping. The work goes on. Full force," Surfside Mayor Charles Burkett said.

Hundreds of emergency responders, including experts from as far away as Mexico and Israel, are assisting with the operation.

Officials say rescuers are removing debris "piece by piece" and using scanners, cranes and dogs to try and find anyone that may still be alive.

Miami-Dade Fire Chief Alan Cominsky said in the latest news update that some 3,000,000lb (1,360,000kg) of concrete had already been removed.

President Joe Biden is expected to visit the building collapse later this week along with first lady Jill Biden.

The president will thank emergency services and meet with families impacted by the "terrible tragedy", White House Press Secretary Jen Psaki said.