A Washington-based research group says North Korea has likely restarted a plutonium reactor, possibly making good on a pledge to expand its nuclear weapons program.
The U.S.-Korea Institute at Johns Hopkins University based its conclusion on recent satellite photos that appear to show white steam emerging from a building near the reactor at the North's main Yongbyon nuclear complex.
The reactor was shut down in 2007 under an aid-for-disarmament deal. But in April, Pyongyang warned it would restart all operations at Yongbyon to boost its nuclear force in both "quantity and quality."
The report, which was published by the institute's 38 North blog, says the color and the volume of the white steam seen in the August 31 commercial satellite photos suggest the reactor is now running.
South Korean Defense Ministry spokesman Kim Min-seok would not confirm the report.
"The South Korean and U.S. governments are keeping a close watch on North Korea's nuclear issue. The picture was taken by a satellite by 38 North, so there's no smoke without fire. You can think of it like that."
James Acton, a senior associate in the Nuclear Policy Program at the Carnegie Endowment for International Peace, says he does think the plutonium reactor has been restarted.
But he tells VOA it could be some time before the reactor is able to produce plutonium that can be used in a weapon.
"You normally need to cook nuclear fuel, as it were. Typically the North Koreans tend to cook it for about a year to produce plutonium before they remove the fuel and then extract the plutonium."
Acton says, when operating normally, the reactor will be able to produce enough plutonium to make about one or two bombs per year. He says Pyongyang is believed to already have enough to make up to 10 bombs.
North Korea is also thought to be pursuing a second, and potentially easier, method of producing nuclear weapons, using uranium, though little is known about how much this program has advanced.
Pyongyang conducted its third nuclear test in February. The United Nations responded by expanding sanctions against the North, which proceeded to launch weeks of angry war threats against the United States and South Korea.
North Korea has since reduced its war-like rhetoric and even suggested the resumption of six-party nuclear talks it abandoned in 2009. The U.S., however, says it will not resume the talks unless Pyongyang shows willingness to abandon its nuclear weapons.
The development comes as the top U.S. envoy on North Korea, Glyn Davies, visits Northeast Asia. Davies said Tuesday in Seoul that Washington remains open to diplomacy, but that North Korea must first be open to "fulfilling its obligations" under U.N. sanctions.