A raging wildfire in the southwestern United States is moving closer to a key nuclear weapons laboratory, forcing it to shut down for another two days.

The website for the Los Alamos National Laboratory in northern New Mexico said only essential personnel will have access to the facility.

Lab officials have said all nuclear and hazardous material – including hazardous waste stored on the complex – is safe.

Crews say the fire has now grown to more than 24,000 hectares. They said the main blaze sparked a smaller fire on the lab site itself, but that it was quickly extinguished.

Officials first evacuated the lab Monday after the wildfire burned through some nearby communities. Thousands of residents have been evacuated and other nearby communities have been put under a voluntary evacuation due to concerns about poor air quality.

Fire crews across the U.S. are currently battling more than 30 large wildfires in nine states, including 14 in Texas, four in Arizona and six in Florida.

Meanwhile, some U.S. nuclear officials are downplaying concerns about two nuclear power plants in the central U.S. state of Nebraska that are being threatened by floodwaters.

Waters from the rising Missouri River have surrounded the Fort Calhoun Nuclear Power Station, while floodwaters have been creeping closer to the Cooper Nuclear Power Station.

The officials that oversee both power plants have said there is no possibility of a meltdown at either facility. U.S. Nuclear Regulatory Commissioner Gregory Jaczko says there is no immediate threat.

There have been concerns that the floodwaters could knock out the plants' electric power supply, allowing the nuclear materials to overheat, leading to a possible meltdown.

Officials say sufficient back-up systems are in place to make sure there is no repeat of the problems that sparked a meltdown at Japan's Fukushima Da-ichi plant following an earthquake and tsunami earlier this year.

Heavy flooding in the central state of North Dakota has threaten dozens of cities across the state.

The Souris River crested at its highest level in 130 years on Sunday, damaging an estimated 3,000 homes and displacing more than 12,000 people in the city of Minot.

The river, which travels from Canada southeast into North Dakota and north back to Canada, has prompted officials in the Canadian province of Manitoba to warn residents to prepare for floodwaters arriving from North Dakota.