Thirteen workers were exposed to radiation in a recent leak at the US Carlsbad underground nuclear waste dump, the Department of Energy admitted .

The department and the contractor that runs the Waste Isolation Pilot Project said in a joint news release that they had notified the affected workers of the positive test results and will do further testing.

"It is important to note that these are initial sample results," the statement said.

"These employees, both federal and contractor, will be asked to provide additional samples in order to fully determine the extent of exposure."

All employees who were working when the leak occurred on February 14 were checked for external contamination before being allowed to leave, the news release said.

But biological samples were also taken to check for possible exposure from inhaling radioactive particles.

Elevated radiation levels have been detected in the air around the plant but officials claim the readings are too low to constitute a public health threat.

The accident is the first release of radiation since the dump began taking plutonium-contaminated waste from the nation's nuclear bomb-building sites 15 years ago.

Officials said they can tell from their analyses of air samples that a container of waste leaked but it could be weeks before they can get underground to find out what caused the problem.

WIPP is the only deep underground nuclear repository in the US and the only facility that can store plutonium-contaminated clothing and tools.