The Waste Isolation Pilot Plant, the nation's only underground nuclear waste repository near Carlsbad, N.M., remains idle on Thursday, March 6, 2014. AP Photo/Susan Montoya Bryan ALBUQUERQUE, N.M. (Reuters) - The planned March 2016 reopening of an underground nuclear waste dump in New Mexico has been pushed back indefinitely because of unanticipated challenges, U.S. officials said.

A radiation leak at the U.S. government's Waste Isolation Pilot Plant that originated in a disposal chamber half a mile (1 km) below ground at the center near Carlsbad, New Mexico, exposed more than 20 workers to small amounts of radiation in February 2014, officials have said.

The accident led to the suspension of key operations at the site, the Energy Department's only permanent underground repository for certain types of radiological waste tied to U.S. nuclear labs and weapons sites.

Dana Bryson, acting manager for the Department of Energy's Carlsbad Field Office, said in a statement on Friday: "We are disappointed that we will not meet the original target date for beginning waste emplacement."

He did not provide a date for reopening the facility.

"While the WIPP recovery program continues to make significant progress, the original target date of March 2016 for resuming waste emplacement operations is no longer viable due to a variety of unanticipated issues," said a news release from the U.S. Department of Energy that contained Bryson's statement.

Key challenges that remain include the need to implement heightened safety standards from the Department of Energy and to resolve problems with the ventilation system, officials said.

"The department is committed to resuming operations at the Waste Isolation Pilot Plant as soon as it is safe to do so," Bryson said.

(Editing by Alex Dobuzinskis)