The UN nuclear watchdog chief and the West, meanwhile, voiced deep concern about North Korea’s expanded nuclear program on Thursday, with Washington warning that the secretive state may be hiding even more atomic sites.

North Korea boasted about its nuclear advances earlier this week, saying it was operating a uranium enrichment plant with thousands of centrifuges. If confirmed, such a facility could offer it a second pathway to make a nuclear bomb.

“These developments ... are a clear manifestation of the risks posed by North Korea’s defiance of its international obligations and commitments,” US envoy Glyn Davies told an International Atomic Energy Agency (IAEA) meeting in Vienna.

Rattled and furious after Pyongyang’s deadly artillery attack, South Korea has moved more troops and guns on to its front-line islands with the North, determined to show it will strike back hard if provoked again.

That threat is real, the country’s spy chief warned, following the North’s Nov. 23 shelling of the island of Yeonpyeong which killed four people and fanned fears of a military escalation on the Korean peninsula. “The danger of further attacks from North Korea is high,” National Intelligence Service director Won Sei-Hoon was quoted as telling a closed session of parliament’s intelligence committee on Wednesday.

He said the North struck at a time of heightened volatility as its leader, Kim Jong Il, is planning to hand power to his 27-year-old son, Kim Jong Un.

Satellite images indicate North Korea may also have suffered “considerable casualties” when South Korea returned fire during the Nov. 23 attack, a lawmaker said, citing satellite images which appeared to show one shell had hit the back of an army barracks.

US envoy Davies told the IAEA’s 35-nation governing board that it was likely Pyongyang had been pursuing enrichment capability before April 2009, the date it most recently claimed.

“If so, there is a clear likelihood that (it) has built other uranium enrichment-related facilities in its territory,” Davies said.

Details about North Korea’s uranium enrichment activities surfaced last month in a report by a US nuclear scientist who had been invited to the country. He said he had seen hundreds of centrifuges and had been told there were more, which the North Koreans said were operating. Centrifuges are devices that spin at supersonic speed refine uranium so that it can serve as fuel for nuclear power plants or, if refined to a much higher degree, for atomic bombs.

North Korea said its enrichment work was aimed at atomic power production, but it already has a plutonium-based nuclear device and carried out two nuclear test explosions in 2006 and 2009. So far it has not shown that it has a working, deliverable nuclear bomb.

IAEA Director General Yukiya Amano expressed “great concern” about reports of the new uranium enrichment facility and construction of a light water reactor in North Korea.

The country has refused full IAEA oversight since 2002 and expelled inspectors last year, leaving the Vienna-based agency with little means to assess its latest nuclear claims or to pressure it into opening up its work to UN monitoring.

The nuclear revelations compounded tensions on the Korean peninsula which have been running high since Pyongyang fired an artillery barrage at a South Korean island, killing four people.

At the IAEA meeting, the European Union urged North Korea to halt construction work on its nuclear facilities and to “refrain from taking any further actions that would increase tensions in the region.”

Some analysts say the attack was Pyongyang’s attempt to force the resumption of international negotiations that could bring it aid.