SEOUL/BEIJING South Korea said on Tuesday it suspects the North has been secretly enriching uranium at more locations besides its main nuclear site, which could mean it has more material for building nuclear bombs.
South Korean Foreign Minister Kim Sung-hwan said he could not confirm a media report that Pyongyang had three to four plants to enrich uranium but he suspected there were facilities in the North in addition to the Yongbyon nuclear complex.
"It is a report based on what is still intelligence and let me just say that we have been following this issue for some time," he told a press briefing.
The prospect of more plants capable of producing materials that could be used in a nuclear weapons program raises the risk that North Korea expands its nuclear plans as it seeks to wrest concessions and aid from restarting disarmament talks.
Last month, North Korea shelled an island close to a disputed maritime boundary with the South, killing four people and prompting the United States to send an aircraft carrier to join military drills with South Korea in a show of strength.
"Our policies have failed," said Hajime Izumi of Shizuoka Prefectural University in Japan. "The situation has caught fire and we are watching it burn."
U.S. nuclear expert Siegfried Hecker, who visited Yongbyon last month, had already raised concerns that the North had alternative sites for uranium enrichment.
U.S. State Department spokesman P.J. Crowley said Washington had long been concerned about such secret activity by North Korea.
"We're very conscious of the fact that in the recent revelations to American delegations, what they saw did not come out of thin air," he said, referring to Hecker's recent trip.
While China has urged South Korea, the United States, Russia and Japan to restart "six-party" talks with the North on nuclear disarmament, South Korea's allies have refused until Pyongyang gives a firm commitment on nuclear disarmament.
CHINA URGES CALM AND RESTRAINT
Chinese Foreign Ministry spokeswoman Jiang Yu told reporters that the senior Chinese envoy to North Korea, Dai Bingguo, had agreed in talks in Pyongyang that nuclear negotiations needed to resume.
"Both agreed that all sides should exercise calm and restraint, and maintain a responsible attitude to prevent tensions from escalating, playing a positive role in preserving the peace and stability of the peninsula," Jiang said.
Hecker was given a tour of the Yongbyon complex on his visit there in November and saw more than 1,000 centrifuges in a building that officials in Seoul and Washington were aware was a uranium enrichment facility.
Hecker was said to have been "stunned" by how modern and updated the centrifuges looked, unlike the clearly aging nature of the rest of the North's nuclear facilities.
David Albright, president of the Institute for Science and International Security, said the revelations helped fill in parts of the North Korean nuclear puzzle but raise new questions about the extent of the North's activities.
"Intelligence agencies have had a list of suspect sites for a long time. What's happening with the new information is they're zeroing in on sites more and trying to identify where this activity could have been taking place," he said.
Uranium enrichment could give the North a second source of fissile material for weapons on top of its plutonium production program at the Soviet-era nuclear program at Yongbyon, which was frozen under a now-defunct international disarmament deal.
The report of additional uranium enrichment facilities came after Russian Foreign Minister Sergei Lavrov chided North Korea over its nuclear program and condemned an artillery attack on a South Korean island that killed four people last month.
North Korean news agency KCNA again on Tuesday accused Seoul of having "misled public opinion" over the shelling of the island, saying that it was the result of a "provocation" that aimed "to kick off the military clash."
Most analysts do not expect North Korea to launch a new round of attacks any time soon, although the shelling of the island was the first time since the Korean War that it had targeted a civilian area.
Earlier this year, the South said the North had torpedoed one of its naval vessels, the Cheonan, killing 46 sailors, something that Pyongyang denies.
Although tensions have subsided, South Korea on Monday launched a new series of live-firing drills at sea, although these are far away from the so-called Northern Limit Line, the maritime boundary between the countries.
(Additional reporting by Andrew Quinn and Paul Eckert in Washington, Yoko Kubota in Tokyo and Chris Buckley in Beijing; Editing by Andrew Marshall and Eric Beech)