At least six people have been killed in a wildfire that has razed a Hawaiian town, officials said Wednesday, with desperate residents jumping into the ocean in a bid to escape the fast-moving flames.

US Coast Guard officers plucked at least a dozen people from the water as emergency services were overwhelmed by a disaster that appeared to have erupted almost without warning.

"I'm sad to report that just before coming on this, it was confirmed we've had six fatalities," Maui County Mayor Richard Bissen told a press conference.

"We are still in a search and rescue mode and so I don't know what will happen to that number."

Lieutenant Governor Sylvia Luke issued an emergency proclamation and told CNN the hospital system on the island of Maui "was overburdened with burn patients, people suffering from inhalation."

"911 is down. Cell service is down. Phone service is down," she said.

Lahaina, a tourist town of 12,000 on the northwestern tip of Maui, lay in ruins, said Governor Josh Green.

"Much of Lahaina on Maui has been destroyed and hundreds of local families have been displaced," said Green.

Video posted on social media showed blazes tearing through the heart of the beachfront town and sending up huge plumes of black smoke.

"People are jumping into the water to avoid the fire," US Army Major General Kenneth Hara, the state adjutant general, told Hawaii News Now.

The Coast Guard said it had "successfully rescued 12 individuals from the waters off Lahaina" and it was sending other vessels to Maui.

Hara said the strong winds had prevented helicopters from being used to carry out rescues or fight the fires.

Lahaina resident Claire Kent said she had seen her neighborhood burned less than an hour after she fled.

"The flames had moved all the way down to the end of the neighborhood," she told CNN.

"We were pulling out... onto the highway, you look back and there's cars with flames on both sides of the road, people stuck in traffic trying to get out," Kent said, describing the dangerous scene as "something out of a horror movie.

"I know for a fact people didn't get out," she said, adding that homeless people and people without access to vehicles seemed to have been trapped in the town.

- Water on fire -

Kent described how power and cell phone coverage had been knocked out Tuesday, and she had been unaware of any approaching danger until a sudden shift of wind sparked panic.

"It was all just word of mouth, like people running down the street saying 'you need to get out.'

"There were guys riding around on bicycles, just screaming at people to leave."

For those who managed to flee, there was worry over missing family.

"I still don't know where my little brother is," Tiare Lawrence told Hawaii News Now. "I don't know where my stepdad is.

"Everyone I know in Lahaina, their homes have burned down," Lawrence said.

Chrissy Lovitt told the outlet every boat in Lahaina Harbor had burned.

"It looks like something out of a movie, a war movie," Lovitt said. "The water was on fire from the fuel in the water."

Surf school manager Elizabeth Smith said she was safe in upcountry Kula, but was concerned for staff and their families in Lahaina.

"We do know that one couple was able to evacuate, but we don't know about the others," she told AFP by telephone.

"There's still very limited communication with people from Lahaina."

- Winds up to 80 mph -

Luke, the lieutenant governor, said the fires were caused by dry conditions and the powerful winds from Hurricane Dora, which is hundreds of miles south of the islands and is not expected to make landfall.

She said the fires have burned hundreds of acres and were being fanned by winds up to 80 miles (130 kilometers) per hour.

Almost 13,000 people were without power on Maui, according to tracking website poweroutage.us.

President Joe Biden said he was mobilizing all resources to help.

"I have ordered all available Federal assets on the Islands to help with response," Biden said.

"The Hawaiian National Guard has mobilized Chinook Helicopters to help with fire suppression and search and rescue on the Island of Maui.

"The US Coast Guard and Navy Third Fleets are supporting response and rescue efforts."