At full power, the five-megawatt reactor at North Korean Yongbyon complex can yield six kilos (13 pounds) of plutonium a year, which is enough for one nuclear bomb.

Satellite images of the complex, taken between January and April, show several "signatures" of low-level activity, experts from the Washington-based Institute for Science and International Security said Thursday.

The signs include irregular snow melting patterns that indicate the inside of the building may have been hot. The experts also highlighted images of a weak stream of warm water being discharged from the reactor, as well as what appeared to be steam from the turbine building.

While the images "do not show clear evidence that the reactor has resumed full power operation," the US think tank said the pictures did suggest the reactor "may be operating at low power or operating intermittently."

The authors behind the latest analysis, David Albright and Serena Kelleher-Vergantini, claimed that the pictures also suggested that a centrifuge plant at Yongbyon nuclear complex had been operated, and that North Korea may be preparing to carry out renovations at the plant.

North Korea mothballed the Yongbyon reactor in 2007, under conditions of an aid accord. However, Pyongyang began renovating the facility in 2013, after the latest nuclear test.

Concern over nuclear arsenal

In February, Albright was among experts at the US-Korea Institute who presented three scenarios about North Korean nuclear arsenal predicting it could grow to 20, 50 or 100 weapons within five years.

However, the Wall Street Journal reported last week that Chinese experts have warned that North Korea may already posses 20 nuclear warheads. In addition, Pyongyang may be capable to create enough weapons-grade uranium to double its arsenal by next year, according to the newspaper.

Those numbers exceed most of earlier US estimates, which claimed that Pyongyang had between ten to 16 nuclear bombs available.

Earlier in April, commander of the US Northern Command and North American Aerospace Defense Command , Admiral William Gortney, said the US military believes North Korea can miniaturize a warhead and put it on a ballistic missile, although there had been no tests.

dj/sms (AFP, Reuters)