SEOUL, South Korea — Two U.S. experts say North Korea is building a new light-water reactor at its main Yongbyon atomic complex, suggesting the secretive regime is following through on a plan to construct a nuclear power reactor, a private American security institute said Saturday.

The Washington-based Institute for Science and International Security last week released commercial satellite images showing a rectangular structure under construction, which it believes is a 25- or 30-megawatt light-water reactor.

“”It’s always disturbing when North Korea starts to build another nuclear reactor or facility,”” said David Albright, institute president. “”Right now, we don’t know if it’s going to be used for civil purposes to make electricity or an attempt to make weapons-grade plutonium.””

The institute was also briefed by two U.S. experts who recently returned from Pyongyang and who were allowed to visit the plant. North Korea said in March that it would build a light-water reactor that would use its own nuclear fuel.

The pair of experts — Siegfried Hecker, former director of the U.S. Los Alamos Nuclear Laboratory, and Jack Pritchard, a former U.S. envoy for negotiations with North Korea — told institute officials that “”the new construction seen in the satellite imagery is indeed the construction of the experimental light-water reactor,”” according to the institute’s website.

North Korea’s nuclear ambitions had been the focus of six-party talks involving China, Russia, South Korea, Japan and the U.S. After stepping away last year, Kim Jong Il’s regime recently has suggested that it wants to return to the bargaining table.

The U.S. and South Korea maintain that North Korea must show its sincerity to disarm before those talks can continue.

North Korea, which maintains an arsenal of atomic weapons, conducted nuclear tests in 2006 and 2009, drawing international concern and sanctions from the United Nations Security Council.

After several years of denying that it was developing such technology, North Korea in 2009 acknowledged that it was in the ultimate stages of uranium enrichment.

Albright said Hecker and Pritchard would make a presentation on their findings on Tuesday at the Korean Economic Institute in Washington. He said the fact that two U.S. officials were allowed to visit the plant suggested Pyongyang was seeking to gain leverage before entering another round of nuclear talks.

Light-water reactors are most often used to make electricity for civilian use but can also be used to enrich uranium, Albright said.