Rescuers in Jersey on Sunday gave up hope of finding survivors from an explosion that flattened a three-story apartment block, confirming five deaths and saying another four people are thought to be lying under rubble.

Police said at least five people were confirmed killed in the blast early Saturday in the Channel island's port capital St Helier, following a suspected gas leak.

"The number of Islanders confirmed to have been killed in the blast is now five," said police chief Robin Smith after earlier giving a provisionally tally of three dead and around 12 more unaccounted for.

"There are a number of residents — we're working on the assumption of four — that are unaccounted for," Smith added in a statement after confirming confirming the search-and-rescue operation had been moved to a recovery operation.

The next phase would involve a "meticulous and painstaking search of the debris," which could take weeks.

A gas leak was the "likely" cause, he said, adding: "But as you often hear the police service say, we keep all our options open."

Aided by sniffer dogs and urban rescue experts from southern England, Jersey emergency workers had picked through the rubble overnight.

'Horribly Wrong'

Firefighters were called out to investigate the smell of gas on Friday evening, more than seven hours before the explosion fire chief Paul Brown.

He conceded that something had gone "horribly wrong" given the subsequent disaster, but insisted Jersey islanders could still have confidence in his service.

"We have been searching and we will not stop searching for people's loved ones," Brown said.

Jersey's gas supplier, Island Energy, said it was working with the fire service to understand what happened.

Like many others in St Helier, Jersey's Chief Minister Kristina Moore said she was woken by the blast.

"Across the island you could hear this extraordinary sound," she told the BBC.

"It's unthinkable news. We're all absolutely devastated and really concerned for the people who are involved, for those whose lives have been lost."

Around 40 residents near the destroyed apartments had to be relocated on Saturday, said officials.

The incident caps a tragic week for Jersey, a British Crown dependency not part of the United Kingdom, whose economy relies on banking, tourism and fishing.

The Jersey Coastguard Friday gave up a search for three Jersey fishermen missing for nearly 36 hours after their wooden boat hit a cargo ship and sank.

The freighter is owned by Condor Ferries, whose Jersey offices lie near the destroyed apartment block.

"We must call on the collective strength of the island community," Moore said after the twin disasters.