SYDNEY (AP) — Three American crew members died today when a C-130 Hercules aerial water tanker crashed while battling wildfires in southeastern Australia, officials said.

New South Wales Premier Gladys Berejiklian confirmed the crash deaths in the state’s Snowy Monaro region as Australia attempts to deal with an unprecedented fire season that has left a large swath of destruction.

Coulson Aviation in the U.S. state of Oregon said in a statement that one of its Lockheed large air tankers was lost after it left Richmond in New South Wales with retardant for a firebombing mission. It said the accident was “extensive” but had few other details.

“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,” said Rural Fire Service Commissioner Shane Fitzsimmons.

He said all three aboard were U.S. residents.

“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.

The tragedy brings the death toll from the blazes to at least 31 since September. The fires have also destroyed more than 2,600 homes and razed more than 10.4 million hectares (25.7 million acres), an area bigger than the U.S. state of Indiana.

Coulson grounded other firefighting aircraft as a precaution pending investigation, reducing 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.

Australian Transport Safety Bureau, the national air crash investigator, and state police will investigate 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.

Berejiklian said there were more than 1,700 volunteers and personnel in the field.