By Laura Zuckerman

(Reuters) - Thirteen workers have tested positive for radiation exposure tied to an accidental release earlier this month of high levels of radiation in an underground nuclear waste repository in New Mexico, the U.S. Department of Energy said on Wednesday.

No workers were underground at the Waste Isolation Pilot Project in southeastern New Mexico when air sensors half a mile below surface in an ancient salt formation triggered an alarm on February 14 indicating excessive amounts of radioactive particles.

The plant is a repository for so-called transuranic waste shipped from federal nuclear laboratories and weapons sites. The waste includes discarded machinery, clothing and other items contaminated with plutonium or other radioisotopes heavier than uranium.

Particles emitted from the decay of those radioactive elements can harm humans if inhaled or ingested.

Managers of the government site near Carlsbad initially said that none of the 139 employees working above ground when the release happened were exposed to radioactive contaminants based on external testing of their skin and clothing.

But analyses released on Wednesday of biological samples lifted from the workers showed that 13 of them were in fact exposed to radioactive particles, Joe Franco, manager of the U.S. Energy Department field office that oversees the plant, said in a statement.

"It is premature to speculate on the health effects of these preliminary results or any treatment that may be needed," he said. He did not give details on the level of contamination detected in the biological samples.

Franco added that continued sampling and monitoring at the site indicated that contamination of surface air in and around the facility was likely at very low levels and not a threat to humans or the environment.

(Editing by Cynthia Johnston and Lisa Shumaker)