The top US nuclear research lab was ordered to stay closed Tuesday after flames reached it and the local town began evacuating, though all high-risk materials were safe, officials said Monday.

The Los Alamos National Laboratory (LANL) said there had been "no off-site releases of contamination" after a one-acre (.40 ha) blaze on the southwestern edge of the research complex northwest of Santa Fe, New Mexico.

An emergency operations unit remains working at the high-security site, founded during World War II when it began developing nuclear weapons technology, and which nowadays employs some 11,800 people.

The center was ordered closed Monday after the Las Conchas fire neared the edge of the site, and in an afternoon update authorities said it would remain closed for at least another day.

"The Lab will remain closed on Tuesday .. because of risks presented by the Las Conchas wild fire and the staged, mandatory evacuation of the Los Alamos town site," where some 42 percent of its staff live, said the LANL website.

Lab boss Charles McMillan said it had been "a very long night for the fire crews" fighting to contain the huge blaze, but the decision to evacuate some 12,000 residents of Las Alamos town was taken after the wind changed direction.

A one-acre spot fire was reported in Water Canyon on the Lab's southwestern boundary, after the blaze jumped across New Mexico State Route 4, said the online update Monday afternoon.

"About one acre burned and the Lab has detected no off-site releases of contamination," it said, noting that the Las Conchas fire had burned some 49,000 acres in forests, canyons, and mesas south and west of the Lab.

"No other fires are currently burning on Lab property, no facilities face immediate threat, and all nuclear and hazardous materials are accounted for and protected."

And it added: "Environmental sites are being monitored and air quality experts are coordinating with the US Environmental Protection Agency."

The Los Alamos national security lab is operated by a partnership between the University of California, Bechtel National, the Babcock & Wilcox Company, and the Washington Division of URS for the US Department of Energy.

Separately, US nuclear authorities were watching floodwaters threatening a nuclear power plant in Nebraska after a protective barrier collapsed.

The 2,000-foot (607-meter) long barrier, holding back floodwaters from the Missouri river, collapsed early Sunday, threatening the Fort Calhoun nuclear power plant.

"This allowed floodwaters to surround the auxiliary and containment buildings, which are protected by design to a floodwater level of 1014 mean sea level," said the US Nuclear Regulatory Commission.

The Missouri River was at 1,006.3 feet (306.7 meters), but river levels were not expected to exceed 1,008 feet, it added in a statement.

The collapse "also allowed floodwaters to surround the main electrical transformers," it said, adding that operators transferred power from offsite sources "as a precautionary measure."

The plant has been shut down since April 7 for refueling, it said.

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