Crews in Hawaii have all but finished searching for victims of the deadliest US wildfire in more than a century, and it is unclear how many people perished.

Three weeks after the fire devastated Maui's historic seaside community of Lahaina, the death toll stands at 115. But an unknown number of people are still missing.

Officials suggested responders likely have already recovered any remains that are recognisable as such, and they are shifting the response to focus on removing hazardous waste and making the area safe for residents to begin returning.

"We have wrapped up almost completely the search and recovery mission and moving into the next phase (hazardous waste removal)," Darryl Oliveira, the interim administrator of the Maui Emergency Management Agency, said on Tuesday.

Maui Police Chief John Pelletier said urban search and rescue teams have "completed 100 per cent of their area" but some search activity continues in the ocean off Lahaina.

The FBI is searching 180m out along a 6.5km stretch of coastline, but no human remains have been found. There are 110 missing persons reports filed with Maui police, and more than 50 of those remain open cases that are still actively being worked.

Although the initial land search is complete, authorities may also use details from the missing person reports to go over areas again.

So far, authorities have identified and notified the loved ones of 45 of those killed. They have collected DNA from 120 people and continue to see more samples.

Maui County didn't sound its emergency sirens to warn residents of the fire when it spread on August 8.

Darryl Oliveira, who took over as interim administrator of the Maui Emergency Management Agency on Monday, said sirens had not been used for wildfires in Hawaii before. But he said Maui, the state, and other Hawaii counties were working on new protocols to do so.

"We see tremendous value in incorporating more tools into our arsenal to keep the public aware and get them early warning," he said, adding that the current hurricane season is overlapping with extremely dry conditions on Maui and statewide.

In an early example of this, officials on Saturday sounded emergency sirens to warn residents of a brush fire near Kaanapali resort hotels in Lahaina. Firefighters quickly brought the fire under control.

Oliveira guided the Big Island through hurricanes and volcanic eruptions as the head of Hawaii County civil defence for many years.

He said one challenge was educating the public, including tourists, on what to do if they hear a siren sound to warn of a wildfire.