SEOUL (AFP) - New satellite imagery suggests North Korea may be firing up a facility for extracting weapons-grade plutonium from reactor waste, a United States think-tank said on Thursday.

The images show steam rising from a reprocessing plant at the North's main Yongbyon nuclear complex - a sign consistent with maintenance and testing prior to commencing operations, the US-Korea Institute at Johns Hopkins University said on its closely followed 38 North website.

The facility is used to reprocess spent fuel from the 5MW reactor at Yongbyon that is North Korea's main source of weapons grade plutonium. The latest satellite pictures indicate the reactor has been shut down for 10 weeks - longer than required for routine maintenance.

While warning it was still early to reach a definitive conclusion, the institute said evidence suggested the shutdown may have allowed the removal of "a limited number" of fuel rods for possible reprocessing. The images also showed truck activity near the vehicle door to the building that receives the spent fuel at the reprocessing complex, it said.

The new analysis comes as Pyongyang is threatening a new nuclear test in reaction to the United Nations adoption of a landmark resolution that condemns North Korean rights abuses and recommends its leaders face charges of crimes against humanity.

North Korea has conducted three nuclear tests, most recently in February 2013.

Pyongyang mothballed the 5MW reactor at Yongbyon in 2007 under an aid-for-disarmament accord, but began renovating it after its last nuclear test in 2013.

When operational, the reactor is capable of producing 6kg of plutonium a year - enough for one nuclear bomb, experts say.

The commander of US forces in South Korea said last month that North Korea had likely mastered the ability to produce a nuclear warhead small enough to be mounted on a missile.