Satellite images suggest North Korea is making plutonium for nuclear weapons, an international security think tank has claimed.

The US-based Institute for Science and International Security said images taken on 30 June show North Korea's five-megawatt-electric (MWe) reactor at the Yongbyon nuclear site continues to be active and that it appears construction and installation work is taking place at the light-water reactor (LWR).

It said several renovations appear to have taken place: "North Korea has apparently made a decision to renovate the aged five-MWe reactor to make plutonium for nuclear weapons for many more years and is expanding the centrifuge plant".

north korea nuclear
The nuclear reactor in Yongbyon Institute for Science and International Security

"The satellite imagery ... suggests that North Korea is emphasising the production of weapon-grade plutonium as well as enriched uranium for its nuclear weapons programme," it added.

The institute said the start-up date for the LWR reactor is unclear, but once up and running it will be able to produce several times more plutonium than the current reactor.

Images showed water being discharged from the five-MWe reactor, suggesting it is operational, but the think tank reported that "without more data, such as regular steam production, it is hard to determine the operational status of the reactor and thus to estimate the amount of plutonium produced by the reactor".

"It is reasonable to assume that North Korea is renovating this reactor so as to achieve the reactor's previous level of plutonium production," the institute added.

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Satellite image showing renovations at the nuclear site Institute for Science and International Security

It is unclear how advanced North Korea's nuclear programme is, making it a matter of concern for world powers.

Last year, state media announced an underground nuclear test had been conducted - the third of its kind in seven years. Japan summoned an emergency UN meeting and South Korea put its military on alert.

Earlier this year, President Obama warned North Korea about testing its nuclear weapons: "North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation. Anybody can make threats. Anyone can move an army. Anyone can show off a missile. That doesn't make you strong.

"We will not hesitate to use our military might to defend our allies and our way of life."