New Mexico's governor has asked President Joe Biden to declare a disaster as firefighters scrambled to clear brush, build fire lines and spray water to keep the largest blaze burning in the US from destroying more homes in the foothills of the Rocky Mountains.

During a briefing on the fire burning across the state's north-east, governor Michelle Lujan Grisham signed a request for a presidential disaster declaration that will be sent to the White House in the hope of freeing up financial assistance for recovery efforts.

She said it is important that the declaration be made now rather than waiting until the fire is out.

"I'm unwilling to wait," said Ms Lujan Grisham, a first-term Democrat who is running for re-election.

"I have families who don't know what the next day looks like, I have families who are trying to navigate their children and healthcare resources, figure out their livelihoods and they're in every single little community and it must feel to them like they are out there on their own."

In the small north-eastern New Mexico city of Las Vegas, residents were already voicing concerns about food stores being closed as some people chose to leave ahead of the flames even though evacuations had not been ordered.

Fire managers told an evening briefing at the local community college that the spread slowed a bit on Tuesday, and put the amount of newly charred land up slightly, to about 231 square miles of mountainsides, towering ponderosa pines and meadows.

Officials have reported about 170 homes destroyed, about 15,500 homes under mandatory evacuation and said the state's psychiatric hospital in Las Vegas remained evacuated. Schools in the community have cancelled classes.

Dan Pearson, a US Forest Service fire behaviour analyst, called Tuesday "a brief reprieve from the extreme conditions we have been experiencing", but warned that winds are expected to increase and shift on Wednesday, pushing fire and smoke towards Las Vegas.

"Tomorrow, we're back to red-flag criteria," Mr Pearson said, adding that forecasts called for better firefighting conditions on Thursday and Friday before winds increase and gusts whip to 50mph or more during the weekend.

Fire engines and their crews were busy working to protect homes and other structures on the edge of Las Vegas while bulldozers cleared more fire lines on the outskirts. Air tanker and helicopter pilots took advantage of a break in the thick smoke and falling ash to drop fire retardant and water.

New Mexico was in the bull's eye for the nation's latest wave of hot, dry and windy weather. Forecasters also issued warnings for parts of Arizona and Colorado, and authorities in Texas urged people there to be careful after crews in that state had to respond to several new fires.

Authorities in north-eastern New Mexico said the flames were a couple of miles from Las Vegas, which serves as an economic hub for most of north-eastern New Mexico and the ranching and farming families who have called the rural region home for generations. It is home to the United World College and New Mexico Highlands University.

The governor said during her briefing that the number of homes destroyed would probably climb much higher given the ground that the fire has covered and the villages that it had moved through over the past week.

San Miguel County officials said on Tuesday they have been unable to get back into burned areas to continue assessments since conditions were too dangerous.

Wildfires have become a year-round threat in the drought-stricken West and they are moving faster and burning hotter than ever due to climate change, scientists and fire experts say.

Fire officials have also said that many forested areas have become overgrown and unhealthy and that the build-up of vegetation can worsen wildfire conditions.