Thousands of residents of Grand Bahama and Abaco islands are without shelter, stranded by flooding and are likely to suffer shortages of food, water and medicine that will worsen without quick action by the international community, according to coordinated messages from the United Nations, the US state department, the US embassy in Nassau and local officials.

“We are in the midst of a historic tragedy,” said the Bahamian prime minister, Hubert Minnis. “The devastation is unprecedented and extensive.”

Minnis announced late in the evening that the death toll had risen to seven, with more deaths expected, and predicted that rebuilding would require “a massive, coordinated effort”.

“We are in the midst of one of the greatest national crises in our country’s history,” Minnis told a news conference. “No effort or resources will be held back.”

The storm hit Abaco island on Sunday as a category 5 hurricane with wind gusts of up to 220mph – making it the strongest Atlantic storm ever to make landfall, tied with the Labor Day hurricane of 1935.

In a country accustomed to harrowing encounters with fierce storms, Dorian registered as a disaster on another scale.

“Dorian will not really move away,” said a tweet from the newspaper in Freeport, the main city on Grand Bahama, on Tuesday morning. “Heavy rain. This hurricane will totally destroy this island.”

The terrible consequences were visible from the first helicopter overflights of Abaco and Grand Bahama islands on Tuesday. Video footage showed lakes of seawater instead of streets, blasted debris where homes once stood, boats thrown inland like discarded bath toys, denuded trees and the occasional lone structure still standing. In many areas, life on the islands appeared to have simply been erased.

“It’s total devastation. It’s decimated. Apocalyptic. It looks like a bomb went off,” Lia Head-Rigby, who helps run a local hurricane relief organization, told the Associated Press. “It’s not rebuilding something that was there; we have to start again.”

Emergency access was threatened by flooding at the Freeport international airport, where runways lay under 6ft of water. The main Freeport hospital was likewise flooded, and an estimated 13,000 homes destroyed.

Flooding drove residents on to roofs, water lapping over second-story windowsills and water driving against shuddering glass. There was waist-deep water in the streets and higher, which residents were struggling to swim or wade through to safety.

Freeport is about 70 miles east of West Palm Beach, Florida, and is a popular cruise ship and tourist destination boasting upwards of 1 million visitors a year.

“The #Bahamas needs our help,” Francis Suarez, the mayor of Miami, Florida, tweeted on Tuesday.

“Our prayers are with our Bahamian friends as they confront the aftermath of Hurricane #Dorian,” tweeted the US secretary of state, Mike Pompeo. The USAID relief agency put out a call for “urgently-needed humanitarian assistance”.