Recent satellite images suggest the nuclear reactor seen as North Korea's main source of weapons-grade plutonium may have resumed low-power or intermittent operations, a US think-tank said Thursday.

Last year, the Washington-based Institute for Science and International Security (ISIS) said the five-megawatt reactor at the North's main Yongbyon nuclear complex appeared to have been shut down -- possibly for renovations.

But satellite pictures taken over the winter period showed several "signatures" of low-level activity, including irregular snow melting patterns on the reactor and turbine buildings, ISIS said in a fresh analysis.

The think-tank also highlighted images of a weak stream of warm water being evacuated from the reactor's discharge pipeline, as well as what appeared to be steam rising from the turbine building.

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

North Korea mothballed the Yongbyon reactor in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.

When fully operational, the reactor is capable of producing around six kilos (13 pounds) of plutonium a year -- enough for one nuclear bomb, experts say.

North Korea carried out nuclear tests in 2006, 2009 and 2013, and there is growing concern that Pyongyang is moving faster down the path towards a credible nuclear deterrent than previously thought.

A recent report by US researchers warned that North Korea appeared poised to expand its nuclear programme over the next five years and, in a worst case scenario, could possess 100 atomic arms by 2020.

And earlier this month, the Wall Street Journal reported that Chinese nuclear experts believed the North may already have a nuclear arsenal of 20 warheads and the uranium enrichment capacity to double that figure by next year.