A wildfire in the US state of New Mexico has moved closer to the Los Alamos laboratory and thousands of outdoor drums of plutonium-contaminated waste.
Authorities have stepped up efforts to protect the site where the first atomic bomb was built and monitor the air for radiation.
Officials at the nuclear weapons lab have given assurances that dangerous materials are safely stored.
They also said they are capable of withstanding flames from the 93sq/km fire, which as of yesterday afternoon was as close as 15m from the grounds.
A small patch of land on the laboratory grounds caught fire on Monday but firefighters quickly put it out.
Teams are on high alert to pounce on any new blazes and spent yesterday 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 said in Los Alamos.
The fire has forced the evacuation of the entire city of Los Alamos, with a population 11,000, and cast giant plumes of smoke over the region.
It has raised fears among nuclear watchdogs that it will reach as many as 30,000 208-litre 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.
Lab officials said there is 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 stand ready to coat the drums with fire-resistant foam if the blaze gets 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 local time yesterday, the flames were about 3km away from the material.