North Korea appears to be moving ahead with the building of a light-water reactor that could be used to support its nuclear weapons program, according to satellite images released Wednesday.

The signs of progress come amid fears that the secretive communist state is planning a new nuclear test, although experts said that the new reactor is likely still years away from being operational.

Commercial satellite images, taken on April 30 and analyzed by Johns Hopkins University's US-Korea Institute, showed advances at the site of a new reactor at the Yongbyon complex with added concrete, steel and roof work.

"It appears that the North may now be nearing completion of the reactor containment building," said a blog entry on the institute's website, 38north.org.

The construction of the reactor, along with a uranium enrichment plant, "is an important indication of the North's intention to move forward with the expansion of its nuclear weapons stockpile in the future," it said.

But it said the facility was unlikely to be operational before 2014 or 2015 as North Korea would still need to install heavy equipment including a turbine generator.

North Korea first disclosed construction on a new reactor in 2010 to visiting US scientists and also showed them a uranium enrichment plant that was said to produce low-enriched fuel for the new facility.

While both were ostensibly for civilian power, the enrichment plant could be converted to produce highly enriched uranium for bombs. Scientists say that the light water reactor could also be run to produce plutonium.

North Korea, whose nuclear program is based on plutonium, has tested two nuclear bombs and has hinted that it may conduct a third explosion.

Young leader Kim Jong-Un's regime said it would suspend nuclear and missile tests along with uranium enrichment under a February 29 deal with the United States.

But the agreement quickly collapsed after North Korea launched a rocket on April 13 in what US officials believe was a disguised, albeit unsuccessful, missile test.