Authorities expected a massive wildfire in eastern Arizona to become the state's largest sometime on Wednesday, as investigators focused on two people whose campfire may have started the blaze and firefighters fought to keep the flames from devouring a small New Mexico mountain town.

Fires also grew elsewhere in New Mexico, including one near the Colorado border that forced the closure of a busy interstate highway.

Kelly Wood, a spokesman for the multi-agency effort battling the Arizona blaze, said late Tuesday that two people of interest were being questioned about an abandoned campfire but he didn't immediately have details. He said the U.S. Forest Service would release more information Wednesday.

In Luna, N.M., just across the Arizona state line, evacuation plans were in place for the roughly 200 residents. Crews have been working to protect the town for days, hacking down brush, using chain saws to cut trees and setting small fires to burn anything that approaching flames could use as fuel.

Fires have devoured hundreds of square kilometres in the southwestern United States and Texas since wildfire season began several weeks ago.

Not much winter precipitation

Scant winter precipitation in Arizona, New Mexico, part of west Texas and southern Colorado is blamed on La Nina, a term describing cooler waters in the equatorial Pacific Ocean that keeps the jet stream from dipping down and bringing storms to the region.

It instead sent rain and snow farther north, which has led to huge snowpack in the Sierra Nevada range in California and in the Rockies.

The wildfire outlook issued by the National Interagency Fire Center in Boise, Idaho, calls for fire potential to be above normal in those areas through September but normal or less than normal across the rest of the West.

The huge blaze in Arizona was also made worse by the extremely thick forest, the result of a century of fire suppression that allowed additional tree growth in the world's largest ponderosa pine forest. Fires that would typically scorch only grasses and small trees on the forest floor across the West now leap into the crowns and skip across miles of terrain through the treetops.

The Arizona fire has burned almost 1,900 square kilometres since Memorial Day weekend and destroyed 32 homes and four rental cabins. It was 20 percent contained as of Tuesday night, Wood said.

Arizona's largest fire was the 2002 Rodeo-Chediski fire, which burned about 1,895  square kilometers but destroyed far more buildings — 491.

While the current fire has surpassed Rodeo-Chediski in size, it still wasn't considered Arizona's largest fire as of Tuesday night because part of it is in New Mexico, where firefighters had intentionally set blazes to keep back uncontrolled flames, said Sean Johnson, another fire spokesman.

Officials expected the blaze in the Apache-Sitgreaves National Forest, which is chewing up thousands of square kilometers a day, to become the state's largest by morning, he said.

"It's just not there yet," Wood added.

Although the fire is enormous, other U.S. blazes between 1997 and 2009 have been much larger.

The biggest was a 2004 blaze in Alaska that burned more than 5,100 square kilometres. The largest in the continental U.S. were a 2006 Texas blaze that consumed 3,670 square kilometres, followed by a 2007 Idaho fire that burned 2,636.61 square kilometres.

2,700 forced from homes

About 2,700 people who live in several Arizona resort communities in the forest remained have been forced to leave their homes.

Wood said residents of the mountain town of Nutrioso would be allowed to return to their homes after 10 a.m. Wednesday, and fire officials said evacuation orders for the picturesque hamlets of Alpine and Greer might be lifted in several days.

Greer, considered the jewel of eastern Arizona's summer havens, lost more than 20 homes and a couple dozen outbuildings as flames moved into the valley last week.

In northeastern New Mexico, a wildfire fanned by high winds has forced hundreds of people from their homes near the Colorado border.

The blaze near Raton was burning on both sides of Interstate 25, closing the highway and sending summer motorists on lengthy detours. Fire officials said at least two structures had burned.