North Korea could be ready within weeks to start operating a light-water reactor that has triggered growing concern amid the regime's vows to build more nuclear weapons, researchers said Wednesday.

The US-Korea Institute at Johns Hopkins University said that satellite pictures taken in March and April appeared to show that North Korea was doing final work inside the reactor and cleaning up after completing construction.

If North Korea has been truthful in its boasts that it has been enriching uranium in a nearby facility since 2010, it may already have enough material to power the reactor for several years, the think tank said.

"This would mean start-up activities could begin in the coming weeks," researchers Jeffrey Lewis and Nick Hansen wrote on the institute's blog, 38 North.

North Korea would still need nine months to a year for the plant to become fully operational, they said.

The light-water reactor would ostensibly provide energy to the resource-poor nation. But the reactor could also be used to produce plutonium for nuclear weapons, which North Korea has vowed to bolster.

The researchers also voiced concern about safety, considering question marks on the level of North Korea's expertise.

"As the Fukushima event in Japan demonstrated, even a well-designed, constructed and tested plant must be capable of addressing unanticipated contingencies such as natural disasters. It is unclear whether the North can deal with such events," they wrote.

The estimate for the start time is earlier than previous private assessments.

The latest findings came after North Korea under young leader Kim Jong-Un vowed to attack the United States with nuclear weapons as part of a showdown with Washington and South Korea.

North Korea carried out its third nuclear test in February. It first revealed work on the light-water reactor when US scientists went on a private visit to the Yongbyon site in 2010.

South Korea described the researchers' report as "worrying" and said that it was necessary to confirm whether it was really a light-water reactor.

"If it's a light-water reactor, there would be considerable restraints on using it to develop nuclear weapons," Defence Ministry spokesman Kim Min-Seok told journalists in Seoul.

"But if it's some other kind of reactor, it could be used to produce more plutonium," he said.