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 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.

A 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.