Stepped up activity at North Korea's main nuclear complex could point to fresh plutonium production to expand its nuclear weapons stockpile, a US think tank said Wednesday.

Fresh satellite images of the Yongbyon facility show high-level activity at two sites, including the five-megawatt reactor seen as North Korea's main source of weapons-grade plutonium, according to analysts at the US-Korea Institute at Johns Hopkins University.

While stressing the difficulty in determining a precise reason for the movement, the analysts suggested a number of scenarios, including renovation work or the replacement of contaminated equipment.

The activity may also indicate preparations for unloading spent fuel rods from the reactor for the purpose of producing new plutonium.

"If this explanation proves to be true, it would represent an important step towards the further development of North Korea's nuclear weapons stockpile," the analysts said in a post on the institute's 38North website.

The increase in activity at Yongbyon was also noted earlier this week by the head of the UN's International Atomic Energy Agency, Yukiya Amano.

"These appear to be broadly consistent with (North Korea's) statements that it is further developing its nuclear capabilities," Amano told members of the IAEA Board of Governors in Vienna.

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.