Officials in Arizona are concerned a massive wildfire, which has destroyed more than 30 homes and forced nearly 10,000 people to flee, could spread definitively into New Mexico over the weekend.

Smoke from the fire was already reaching into central New Mexico on Saturday and has prompted officials to warn residents as far away as Albuquerque about potential respiratory dangers.

The fire is being pushed by strong winds, and was threatening more towns on Saturday and could endanger two major power lines that bring electricity from Arizona to West Texas.

Small spot fires ignited across the border in New Mexico, but firefighters moved in to counter them.

The wildfire, which remains largely uncontained, has burned 1,740 square kilometres of forest, an increase of 380 square kilometres from a day earlier, officials said.

"It's getting very, very close to the New Mexico state line," Jim Whittington, spokesman for the teams battling the fire. "This is really rugged country.

"The atmosphere will be unstable and very conducive to fire growth. We're very concerned about the winds."

Officials have called it the second biggest fire in the state's history, with more than 4,400 firefighters attacking the blaze.

Sooty air dangerous for health

The fire has officials worried about serious health impacts to residents and firefighters as tiny particles of soot in the air reached "astronomical" levels. Those levels were nearly 20 times the federal health standard on Saturday, but the good news was that was down from roughly 40 times higher a day earlier.

Sunday could get even worse, said Mark Shaffer of the Arizona Department of Environmental Quality. "Things got better but they're still bad," Shaffer said Saturday.

The microscopic particles, about 1/28th the width of a human hair, can get lodged in the lungs and cause serious health problems, both immediate and long-term, Shaffer said.

Officials planned to bring in more air quality monitoring equipment over the weekend, and warned people to just stay away. Meanwhile, investigators looking into the cause of the wildfire are focusing on two small areas in the Bear Wallow Wilderness where smoke and flames were first seen on May 29. The two fires merged into one, runaway blaze, which as of Saturday afternoon was only five per cent contained.

More than 30 homes have been destroyed since then, and nearly 10,000 people have been evacuated from the two Arizona towns of Eagar and Springerville and from several mountain communities in the forest.