Federal help was set to arrive on Monday, local news site the Arizona Republic reported, but officials expected at least 250 homes -- about half the town of Yarnell -- would be destroyed by the inferno.
"This is as dark a day as I can remember," said Arizona governor Jan Brewer, as tributes poured in for the fallen firefighters.
Speaking to a local NBC affiliate, she added: "When we get these drought areas, sometimes it dries out, it just burns so quickly and so fast and tonight is another example of that.
"I guess it was a flash fire and it just turned around and it overcame them all. It was just one big swoop," she told KPNX-TV.
A state forestry official told CNN the firefighters, from a local "hotshot" crew, had been digging a fire line to contain the blaze.
"In normal circumstances, when you're digging fire line, you make sure you have a good escape route, and you have a safety zone set up," Art Morrison said.
"Evidently, their safety zone wasn't big enough, and the fire just overtook them."