Sydney, Australia — Three Americans died Thursday when a C-130 Hercules aerial water tanker crashed while battling wildfires in southeastern Australia, officials said. The plane came down in the mountainous Snowy Monaro region of Australia's New South Wales state.

It was operated by a Canadian company, Coulson Aviation. Coulson's Portland, Oregon branch said in a statement that one of its Lockheed C-130s was lost after it left Richmond in New South Wales with retardant for a firebombing mission. It said the damage was "extensive" but had few other details. Coulson said it would send a team to the crash site.

New South Wales Premier Gladys Berejiklian confirmed the deaths and crash in comments to reporters as Australia attempts to deal with an unprecedented fire season that has left a large swath of destruction.

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New South Wales Rural Fire Service Commissioner Shane Fitzsimmons said, "The only thing I have from the field reports are that the plane came down, it's crashed and there was a large fireball associated with that crash." He said all three aboard were U.S. residents.

Undated photo released by the New South Wales Rural Fire Service, a C-130 Hercules water tanker nicknamed "Thor" drops water during a flight in Australia. HOGP / AP

"Unfortunately, all we've been able to do is locate the wreckage and the crash site and we have not been able to locate any survivors," he said.

A spokesman for Australia's Civil Aviation Safety Authority said the aircraft went into a valley to drop the retardant and didn't emerge, according to the Reuters news service.

A receptionist at a hotel near the crash site told Reuters said she heard police cars race by and, "We saw a lot of smoke all of a sudden."

Australia's national air crash investigation agency and state police will comb the crash site, which firefighters described as an active fire ground.

"There is no indication at this stage of what's caused the accident," Fitzsimmons said.

Coulson grounded other firefighting aircraft as a precaution pending the investigation, reducing the number of planes available to firefighters in New South Wales and neighboring Victoria state.

The four-propeller Hercules drops more than 4,000 gallons of fire retardant in a single pass

The tragedy brings the death toll from the blazes to at least 31 since September. The wildfires have also destroyed more than 2,600 homes and razed more than 25 million acres, an area bigger than Indiana.

The wildfires are also estimated to have killed about one billion animals.

Berejiklian said there were more than 1,700 volunteers and personnel in the field, and five fires were being described at an "emergency warning" level — the most dangerous on a three-tier scale — across the state and on the fringes of the nation's capital, Canberra.