LAS VEGAS, N.M. (AP) — Firefighters in New Mexico’s Rocky Mountain foothills prepared Monday to excavate new firebreaks and clear brush to create more defensive lines aimed at preventing a massive wildfire from destroying more homes and tinder-dry pine forests.

The fire that is largest in the U.S. has burned nearly 300 structures including homes, commercial buildings and barns. The tally is likely to be higher since authorities have been unable to access some areas to survey the damage.

The fire jumped a highway late Sunday — taking hold in rugged areas difficult for firefighters to reach and prompting a warning for more residents of rural villages to be prepared to flee quickly.

Another New Mexico wildfire in the mountains surrounding one of the federal government’s key facilities for nuclear research prompted Los Alamos National Laboratory and community officials to prepare for possible evacuations. Officials stressed there was no emergency, but the fire was within about 3 miles (4.8 kilometers) of the lab and was growing.

“If you don’t have to be at work, it’s time to prepare to telework,” lab director Thom Mason told employees in a video. “Conditions can change quickly, it has been very dry, very windy, and we have to be respectful of that risk and ready for what comes next.”

There was no letup Monday to the gusty winds that complicated firefighting efforts over previous days. The wind has fanned the New Mexico fires for weeks with only brief interruptions and the most recent wave of consecutive days of extremely dangerous wildfire conditions are unprecedented, weather forecasters said.

Nearly 1,700 firefighters were battling the biggest blaze burning northeast of Santa Fe, New Mexico. It has charred more than 296 square miles (766 square kilometers), an area nearly the size of New York City. After fighting it for nearly a month, firefighters had contained almost half of the blaze by Monday, a feat that operations section chief Todd Abel said was significant given the challenges crews have faced.

The region’s largest population center — Las Vegas, New Mexico, home to 13,000 people — was declared largely safe from being burned after firefighters mostly stopped the fire on that front. But thousands of people living in smaller, outlying communities were still under evacuation orders.

The northern and southern flanks of the wildfire have proven trickier to contain as wind gusts over the weekend topped 50 mph (80 kph). On Monday, the wind was too strong to launch aircraft to help with the firefighting effort.

The aircraft are used to drop water directly on flames or lay retardant ahead of the blaze’s expected direction so that bulldozers and ground crews can dig firebreaks in places where there are no roads that function as firebreaks.

The National Interagency Fire Center has said more than 20,000 New Mexico structures were threatened by the fire.

Authorities late Sunday told residents in small villages on the northern front of the fire to evacuate, saying it was approaching quickly after jumping a road.

People who wait too long to leave could face life-threatening situations fleeing because of heavy smoke and congested roads, said Dave Bales, the incident commander of the team fighting the fire.

That sort of scenario makes the smoke “so thick you can’t see, you can’t drive, you can’t see the engine ahead of you,” he said.

The threatened communities are along roads that runs from Las Vegas, New Mexico, up to skiing and other resorts in far northern New Mexico. People in some parts Taos County were told to prepare for possible evacuations.

Nationwide, three new large fires were reported over the weekend — two in Arizona and one in Texas.

The National Interagency Fire Center reported Monday that a dozen uncontained large fires have burned more than 503 square miles (1,303 square kilometers) in four states so far this year. Ten of these large fires and nearly all of the acres burned are in New Mexico and Arizona.

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 have said.

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

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Ronayne reported from Sacramento, California. Associated Press writer Scott Sonner in Reno, Nevada, and Susan Montoya Bryan in Albuquerque, New Mexico, contributed to this report.