Firefighters in the US state of Arizona brought in a huge air tanker Thursday to help douse wildfires threatening two towns and nearing the state line with New Mexico, a spokesman said.

Some 6,000 people have been ordered to evacuate from the communities in the path of the blaze, which has now consumed some 140,000 hectares (550 square miles), according to updated figures reducing its estimated size.

Less powerful winds were fueling hope that the blaze, the second biggest in Arizona's history, could be tamed, although for the moment it is burning out of control.

"It's uncontained, but every piece of manpower here is involved in the effort to contain it .. the effort is actually incredible," incident command center spokesman John Helmich told AFP.

The towns of Eagar and Springerville were ordered to evacuate on Wednesday, he said.

"The fire is approaching them, but there are significant resources being used to prevent the fire from rolling into them.

"At this present time that effort has so been successful," he said.

Some 3,000 emergency services staff and nearly 200 fire engines are currently involved in trying to contain the Wallow Fire, while 16 helicopters have been dumping flame retardants from the air.

On Thursday they were due to be joined by a DC-10 plane which can drop some 13,000 gallows of water or retardant at a time, more than four times what the choppers can lift.

The aircraft was due to make a practice flight unloaded to assess flying conditions, before being loaded up to begin dousing the flames.

The blaze was also "close" to Arizona's border with New Mexico, the spokesman said, adding that if it spreads across the state line they could begin calling on resources from the neighboring state.

Although Eagar and Springerville residents were strongly advised to evacuate on Wednesday, the order was not mandatory and some residents were reportedly refusing to leave.

"People will do this and that's their right," said Helmich, but added: "It's never advised on our part to try to hang out."

Arizona Governor Jan Brewer on Monday signed a declaration of emergency in response to the wildfires, releasing $200,000 from an emergency response fund.

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