Fresh satellite images suggest North Korea is expanding its uranium extraction capacity, possibly with a view to increasing its stockpile of weapons-grade fissile material, according to a leading non-proliferation expert.

The images show Pyongyang has begun to refurbish a major mill that turns uranium ore into yellowcake -- a first step towards enriched uranium, said Jeffrey Lewis, an arms control expert at the Monterey Institute of International Studies.

"This suggests that North Korea intends to mine and mill a significant amount of uranium that could serve as fuel for expanding its nuclear weapons stockpile," Lewis said in an analysis posted late Wednesday on 38North, the website of the US-Korea Institute at Johns Hopkins University.

Lewis noted that the yellowcake might also be destined to produce fuel for an experimental light-water reactor under construction at the North's main Yongbyon nuclear complex.

The analysis comes days after experts at IHS Jane's said separate satellite images suggested North Korea was operating a second hall of uranium enrichment centrifuges at Yongbyon.

North Korea has staged three successful nuclear tests in 2006, 2009 and 2013.

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