Unusually high levels of radioactive particles were found at an underground nuclear waste site in New Mexico on Saturday in what a spokesman said looked like the first real alarm since the plant opened in 1999.

U.S. officials were testing for radiation in air samples at the site where radioactive waste, such as plutonium used in defense research and nuclear weapon making, is dumped half a mile below ground in an ancient salt formation.

"They (air monitors) have alarmed in the past as a false positive because of malfunctions, or because of fluctuations in levels of radon (a naturally occurring radioactive gas)," Department of Energy spokesman Roger Nelson said.

"But I believe it's safe to say we've never seen a level like we are seeing. We just don't know if it's a real event, but it looks like one," he said.

It was not yet clear what caused the air-monitoring system to indicate that radioactive particles were present at unsafe levels, Nelson said.

No one was underground at the Department of Energy Waste Isolation Pilot Plant near Carlsbad, in New Mexico's south east, when the alarm went off at 11:30 p.m. MST on Friday, and none of the 139 employees working above ground at the facility was exposed to radioactive contaminants, he said.

Workers were asked to shelter where they were until the end of their shifts and were allowed to leave the facility at 5 p.m. local time on Saturday, Nelson said. No air exchange with the surface was occurring after the ventilation system automatically switched to filtration, he said.

Nelson said the facility may have accurate measurements as early as Sunday on the number of airborne alpha and beta particles, which can be harmful if inhaled or ingested.

A team could be sent below ground before the end of the weekend and Nelson said the plant was "not in active operations. We're in a period we have normally reserved for shutting down the facility for maintenance".

A different part of the site was evacuated this month after a truck used to haul salt caught fire. Several workers suffered smoke inhalation, an agency statement said. (Reuters) 

READ MORE: Possible radiation leak at New Mexico military nuclear waste site