Survivors on Caribbean islands shattered by Hurricane Irma begged the world for food, water, shelter and rescue on Saturday as they faced down armed looters and the prospect of a fresh onslaught from strengthening Hurricane Jose.

In Florida, panic grew as Irma approached. City streets emptied, the skies over the state were quiet after airports closed and those who chose to defy evacuation orders bunkered down for the coming fury.

They have endured a days-long preview of the storm’s ferocity as it crashed into homes, hospitals, schools and airports across the Caribbean, wiping out buildings and infrastructure and claiming 20 lives already, a toll that is almost certain to rise.

The British Virgin Islands declared a state of emergency and brought in a curfew to deal with devastating destruction; there were reports of looting in Franco-Dutch St Martin, one of the first places hit. In many of the islands that took the full force, most homes were damaged or destroyed.

Nick Cunha, a publisher who lives on Tortola, was in one of the houses picked apart by the storm. He had taken shelter with friends who thought their home was stormproof; in fact, the five adults and set of five-month-old triplets cowering inside barely survived.

The house literally blew away, it blew into the road and the hillside behind it Nick Cunha, Tortola resident

“The house literally blew away, it blew into the road and the hillside behind it, and the room that we were in was the only part that stayed still,” said Cunha, who is helping coordinate relief. “Layers of the home were being stripped away and as each one went the next one was taken faster.”

The group retreated into a bathroom, trying to barricade the window with mattresses and the door with chests, but the winds simply ripped off the roof, forcing them to climb out of a tiny bathroom window and shelter beside a generator. When Cunha returned to his own home the next day, looters were already picking through the shattered wreckage. “We have lost everything,” he said.

The group are now sleeping in a bank with another family, rationing their tinned food and drinking water, and bracing for the forecast arrival of Hurricane Jose. That storm may be less fierce than Irma, but the debris still lying in many areas could make it more dangerous. They hope the arrival of Royal Marines will stem the violence and speed up the search for missing friends and family.

Also badly hit were parts of the Turks and Caicos. At least 13 people, including children, have not been heard from since the storm descended on the tiny island of Salt Cay, where they chose to stay in defiance of an evacuation order.

“I am very concerned about Salt Cay; it looks like the southern half has flooded,” said Stacey Schneider, who set up a Facebook page to share information about the crisis, where residents and friends begged the government to send a boat or plane to check on the group.

In Florida, state governor Rick Scott visited Sarasota County on the state’s west coast as Irma moved north from Cuba and edged closer to landfall. He warned that the window of time for evacuation was closing rapidly. “If you have been ordered to evacuate you need to leave now. Not tonight, not in an hour, you need to go right now,” he said.

Irma was expected to strike the Florida Keys very early on Sunday morning. Thousands of people have already left their homes – though, as the Miami Herald reported, some diehards are clinging on – which seems rash bearing in mind the threat of a storm surge of up to 10ft.

In Miami, power outages in 26,000 homes were reported on Saturday. Photograph: Joe Raedle/Getty Images

In Miami, the first power outages – of 26,000 homes – have been reported, but hopes are rising that it may escape the worst of Irma’s fury as the eye of the storm swings towards the west coast.

Nonetheless, the streets were virtually empty, and all petrol stations, supermarkets and other commercial outlets were closed. The city of 6 million people is now in a stunned state of foreboding, waiting to find out what nature has in store.

The storm slammed into Cuba on Friday night, taking many in the country by surprise after veering farther south than forecast. It had regained strength at sea to land as the first category 5 hurricane to hit the island since 1924.

Hundreds of thousands of people in central areas had already been evacuated. Electricity was cut along much of the northern coast, and the storm ripped apart buildings, felled trees, and churned up metres-high waves, as it had elsewhere. Despite the extensive damage, Cuba is sending doctors around the Caribbean, to Antigua and Barbuda, Saint Kitts and Nevis, Saint Lucia, the Bahamas, Dominica and Haiti to help with relief.

On Barbuda there was unexpected joy, after preparations for another hurricane were set aside when Jose changed course. “It’s a good morning,” ABS TV station said on its website. The island, where almost all homes were rendered uninhabitable by the storm, had already been emptied of its 2,000 strong population by a government that didn’t want to take chances.

“There are a few persons who have decided they think they are all right, one of them is actually living in a cave. About maximum 10,” said Arthur Nibbs, minister for Barbuda affairs, who rode out Irma at home but said it had been an unprecedented disaster.

From the Virgin Islands, survivors painted a picture of desolation and violence, but warned that the situation could deteriorate if aid did not arrive fast. “There is nothing left, I don’t think people will believe the severity of what happened. There is no water, no food, no supplies, no transport. You can’t get from one part to another unless you walk,” said Giles Cadman, who lives on the islands part-time and has been part of a remote relief network that sprang up in the wake of the disaster.

He worries that public perception of the Virgin Islands as a millionaire’s playground could undermine promises of relief aid, even though most residents are ordinary workers on modest salaries.

I think the aftermath is proving worse than the hurricane itself Dr Annalise Wilkins, Tortola resident

Dr Annalise Wilkins left her home on Tortola to go to a wedding, and now fears she will never see it again. The island does still have a hospital but, as one of few largely intact buildings, it has been partly commandeered by the government, she said. And it was still badly hit by flooding and wind damage, with windows blown in and the dialysis unit flooded. For now, Wilkins is trying to coordinate relief efforts from a distance, and prepare helicopter transport of urgent medical supplies that are already running low.

Longer term, she is worried about disease and water supplies, with most of the island’s cisterns and all but one of the desalination plants taken out. “Even basic infrastructure is going to take months.”

She fears rebuilding will be slow and the damage may yet intensify. “I think the aftermath is proving worse than the hurricane itself. The devastation and people running out of food and water and not being able to get out.”