A Canadian-owned C-130 Hercules air tanker has crashed while fighting bushfires in Australia's alpine region, killing all three of its crew.
All three victims were American residents, Australian authorities said, adding that they did not immediately know why the plane crashed.
"Tragically, there appears to be no survivors as a result of the crash down in the Snowy Monaro area," said Shane Fitzsimmons, the Rural Fire Services Commissioner for New South Wales state.
He said the tanker "impacted heavily with the ground and initial reports are that there was a large fireball associated with the impact of the plane as it hit the ground. There is no indication at this stage of what's caused the accident."
A spokesman for Australia's Civil Aviation Safety Authority said that the aircraft went into a valley to drop the retardant and did not come out.
Mr Fitzsimmons said the plane had been leased by Canadian aerial firefighting company Coulson Aviation, which had a second C-130 Hercules working in the Australian bushfires operation.
He said Coulson had grounded its large air tanker fleet following the crash, pending a review to ensure there was no systemic issue like a fuel problem.
The Australian Transport Safety Bureau said it was sending a team of investigators to the crash site to collect evidence and would analyse available recorded data, review weather information and interview any witnesses.
It said a preliminary report would be released in about 30 days, but if a critical safety issue was identified during the course of the investigation it would immediately notify the relevant stakeholders.
Coulson said the plane was on a firebombing mission and was carrying a load of retardant. The company said it will send a team to the crash site to assist in the emergency operations.
"The accident is reported to be extensive and we are deeply saddened to confirm there were three fatalities," the company said in a brief emailed statement.
The deaths take the toll from hundreds of wildfires in Australia to 32 since September.
An estimated one billion animals have also died, while the fires have incinerated 2,500 homes and a total area of bushland one-third the size of Germany.
Authorities had previously said the tanker was flying in the Snowy Monaro region, south of the capital Canberra.
Flight tracking website Flightradar24 showed the flight path for an air tanker used in water bombing operations suddenly stopping in Peak View, south of Canberra.
According to its flight data, the aircraft departed the Richmond air force base in western Sydney around 12.15pm. It dropped off radar just after 2pm.
Peak View is close to a blaze burning out of control in the Wadbilliga National Park, the fire service said.
Such air tankers typically carry 15,000 litres of water or fire retardant to be released over fires, which can help contain blazes in areas ground crews find hard to reach.