The only man to survive a fire that killed 19 firefighters in Arizona was moving the crew's truck at the moment the flames engulfed them.

The intense flames roared over the 19 firefighters who were part of a highly trained crew called "hotshots". Eighteen of the men were from the Granite Mountain Hotshots team, while the 19th victim was from another Hotshot crew.

They died despite trying to take shelter in special protection tents designed to deflect the heat.

Commanders have said the hotshot crew had been following safety protocols, but the fire's erratic nature overwhelmed them.

"It's been a long night. And these are the worst of times for firefighters," Commander Roy Hulls said. "We wish circumstances were better."

An investigation is underway into the disaster, the worst loss of life among firefighters in the United States in 80 years.

A report from a helicopter crew flying over the area has said some of the victims made it into their protection shelters, but some did not.

"There was nothing they (helicopter crew) could do to get to them," a former fire district chief who assisted in the early effort to contain the blaze told Reuters.

Crew faced 'perfect storm' of fire intensity

Officials have said the crew faced a volatile mix of erratic winds gusting to gale-force intensity, low humidity, a sweltering heatwave, and thick, drought-parched brush that had not burned in about 40 years.

But they have said those are the conditions such "hotshot" crews, deployed to fight fires at close range with hand tools, train for.

"It had to be a perfect storm in order for this to happen. Their situational awareness and their training was at such a high level that it's unimaginable that this has even happened," Prescott Fire Department spokesman Wade Ward told America's ABC network.

Arizona's governor has ordered state flags to fly at half-mast for two days, calling the deaths "one of our state's darkest, most devastating days".

US president Barack Obama, who is travelling in Africa, issued a statement paying tribute to those who lost their lives in what he called a "terrible tragedy".

"They were heroes - highly-skilled professionals who, like so many across our country do every day, selflessly put themselves in harm's way to protect the lives and property of fellow citizens they would never meet," the statement said.

The blaze was ignited last weekend by lightning near Yarnell, about 128km northwest of Phoenix, and was still uncontained after burning 3400 hectares of bushland.

Authorities ordered the evacuation of Yarnell and the adjoining Peeples Valley. The two towns are southwest of Prescott and home to roughly 1000 people.

A Yavapai County Sheriff's Office spokesman said at least 200 structures had been destroyed, most of them in Yarnell, a community consisting largely of retirees. Fire officials said most of the building lost were homes.

Experts have said the current US fire season could be one of the worst on record.

The disaster in Arizona marks the highest death toll among firefighters from a US wildfire since 29 men died battling the Griffith Park fire of 1933 in Los Angeles. The deadliest day for firefighters overall in the US was when 340 were killed in the World Trade Center on September 11, 2001.

Scorching heat is expected to last for the first part of the week, meteorologists have said.

- with Reuters