SYDNEY, NSW, Australia - Flags will be flown at half mast throughout New South Wales in Australia on Friday, following the devastating crash of a large fire-fighting air tanker in the state on Thursday.

Three U.S. firefighters, helping to battle the Australian bushfires that have ravaged the country in recent weeks, were killed in the crash. They were Captain Ian McBeth, first officer Paul Hudson and flight engineer Rick DeMorgan.

The crash occurred near Cooma, north-east of the Snowy Mountains, in southern New South Wales.

"It's just a ball of flames ... over," another plane flying nearby described the stricken aircraft as in a message to flight control.

New South Wales Rural Fire Service Commissioner Shane Fitzsimmons later described the crash as "a large fireball associated with the impact of the plane as it hit the ground."

"Our thoughts are not just with family and loved ones, but for anyone who feels impacted by what has unfolded this afternoon," he said later Thursday. "We can't thank enough people who continue, notwithstanding the conditions, to put their safety at risk to protect lives and property of others."

New South Wales Premier Gladys Berejiklian added: "Today again demonstrates the fire season is far from over."

Mr Fitzsimmons said Coulson Aviation, the Canadian operator of the C-130 aircraft, had grounded its fleet on Thursday afternoon as a mark of respect for the the fallen firefighters and to review the circumstances of the crash.

"Obviously, grounding of a number of aircraft momentarily will have an effect on our aerial capacity, but it's absolutely warranted and I support them 100 per cent in grounding them for the reasons that they stipulated," he said.

The plane was built in 1981 in the U.S. by Lockheed Martin. It took off from the Royal Australian Air Force base at Richmond, NSW. It vanished from the flight radar just after 2:00pm on Thursday afternoon.

The principals of Coulson are on a flight from Canada and are expected to be in Australia within hours, Mr Fitzsimmons said.

The Coulson family on Thursday night issued a statement. "While working in the Snow Monaro area in southern New South Wales, Australia, contact was lost with one of our large airtankers, a Lockheed C-130 registration N134CG," the statement said.

"The aircraft had departed Richmond, NSW with a load of retardant and was on a firebombing mission. The accident is reported to be extensive, and we are deeply saddened to report there have been 3 fatalities."

"The accident response team has been activated as well as local emergency services, Coulson Aviation will be sending a team to the site to assist in emergency operations," the statement said.

"Our thoughts and prayers are with the families of the three crew members aboard."

A number of crews from the U.S. and Canada, as well as from New Zealand, have joined their Australian counterparts to fight the fires, which are of an unprecedented scale.

"As the Premier indicated, it is a confronting and sobering reminder of the inherent risks associated with firefighting and we've seen all too often this season, unfortunately, the tragic consequences that can come as a result of these significant bushfires that we've seen burning across New South Wales," Mr Fitzsimmons said.

"The crew on board were well known, not just to their colleagues here in Australia, but we're also reminded that a number of our U.S. colleagues that are embedded in some of the incident management teams right now, including down in the high country, actually had personal relationships with them," he added.

"Our hearts are out with all those that are suffering what is the loss of three remarkable, well-respected crew that have invested, you know, so many decades of their life into firefighting and fire management, and are professionals in the aviation firefighting sector."

The U.S. Ambassador to Australia, Arthur B. Culvahouse Jr, said in a statement released on Thursday night: "I am deeply saddened by the tragic news we received today. The brave Americans who died near Snowy Monaro died helping Australia in its time of need."

"The families and friends of those who we have lost are in our thoughts and prayers. Thank you Australia for your sympathy and solidarity."

Helicopters and an air force plane circled the Snowy Monaro region after contact was lost with the large air tanker which was working on bushfires in the area, The Sydney Morning Herald reported.

As Thursday was coming to an end, just before midnight, there were 73 bush and grass fires burning across New South Wales, with thirty not yet contained.

Some relief was expected Friday as cooler weather was forecast.

(File photo. Credit NSW RFS).