A wildfire burning near the desert birthplace of the atomic bomb advanced on the Los Alamos laboratory and thousands of outdoor drums of plutonium-contaminated waste on Tuesday as authorities stepped up efforts to protect the site from flames and monitor the air for radiation.

Officials at the nation's premier nuclear weapons lab gave assurances that dangerous materials were safely stored and capable of withstanding flames from the 240 square kilometer fire, which as of midday was as close as 15 meters from the grounds.

A small patch of land on the laboratory grounds caught fire on Monday before firefighters quickly put it out. Teams were on high alert to pounce on any new blazes and spent the day removing brush and low-hanging tree limbs from the lab's perimeter.

"We are throwing absolutely everything at this that we got," Democratic Senator Tom Udall of New Mexico said in Los Alamos.

The fire has forced the evacuation of the entire city of Los Alamos, with a population of 11,000, cast giant plumes of smoke over the region and raised fears among nuclear watchdogs that it will reach as many as 30,000 208-liter drums of plutonium-contaminated waste.

"The concern is that these drums will get so hot that they'll burst. That would put this toxic material into the plume. It's a concern for everybody," said Joni Arends, executive director of the Concerned Citizens for Nuclear Safety, an anti-nuclear group.

Arends' group also worried that the fire could stir up nuclear-contaminated soil on lab property where experiments were conducted years ago. Over the years, burrowing animals have brought that contamination to the surface, she said.

Lab officials said there was very little risk of the fire reaching the drums of low-level nuclear waste, since the flames would have to jump through canyons first. Officials also stood ready to coat the drums with fire-resistant foam if the blaze got too close.

Lab spokeswoman Lisa Rosendorf said the drums contain Cold War-era waste that the lab sends away in weekly shipments for storage. She said the drums were on a paved area with few trees nearby. As of midday on Tuesday, the flames were away from the material.

"These drums are designed to a safety standard that would withstand a wildland fire worse than this one," Rosendorf said.

Los Alamos employs about 15,000 people, covers more than 93 sq km, includes about 2,000 buildings at nearly four dozen sites and plays a vital role in the nation's nuclear program.

The lab was created during World War II as part of the Manhattan Project to build the atomic bomb. It produced the weapons that were dropped on Hiroshima and Nagasaki.

In the decades since, the lab has evolved into a major scientific and nuclear research facility. It stockpiles aging atomic materials, tests warheads, produces triggers for nuclear weapons and operates supercomputers and particle accelerators.

It also conducts research on such things as climate change and the development of a scanner for airports to detect explosive liquids. The lab's supercomputer was used in designing an HIV vaccine.