It was not clear whether the comments by Kim Tae-hyo, President Lee Myung-bak’s deputy national security adviser, were based on new intelligence.

They followed a report last week by the Washington-based Institute for Science and International Security that satellite images from Sept. 29 showed new construction activity in the area surrounding North Korea’s nuclear reactor.

Kim’s comments were reported Wednesday in the JoongAng Ilbo newspaper. Kim confirmed to The Associated Press that he made the comments Tuesday at a forum on Northeast Asia, but declined to elaborate.

“The North Korean nuclear threat has, in reality, been accelerating and has now reached an extremely dangerous level,” Kim said.

North Korea, which has active nuclear and missile programs, conducted underground atomic tests in 2006 and 2009, drawing tough international sanctions in response.

South Korea, along with the United States, China, Japan and Russia, have been negotiating with the impoverished country since 2003 to get it to dismantle its nuclear facilities, which they consider a threat to regional security.

North Korea, however, pulled out of the talks last year amid an international row over its firing of a suspected long-range missile that the North said was a satellite launch.

“Should North Korea reduce the size of nuclear warheads and use them in actual battle, regardless of their accuracy, they will cause an unbelievable amount of damage,” Kim said.

Most security experts think North Korea remains unable to deliver a nuclear warhead on a missile, but it is believed to be trying to develop this capability and some observers think it may have come close already.

“Although the North Koreans carried out two nuclear tests, analysts in the West doubt that they have successfully loaded warheads onto missiles,” said Kim Tae-woo, a senior research fellow at the state-run Korea Institute for Defense Analysis in Seoul. “But I can say with certainty that they are extremely close. They might have done so already.”

Presidential adviser Kim said that the North is operating not only its plutonium-producing Yongbyon nuclear facility, but also highly enriched uranium activities elsewhere in the country.

He also suggested that there is potential danger in the emergence of North Korean leader Kim Jong Il’s youngest son, Kim Jong Un, as his heir apparent. The younger Kim made his public debut last week after being promoted to four-star general and vice chairman of the ruling Workers’ Party of Korea’s Central Military Commission.

He said Kim was 26 years old and was born on Jan. 8, 1984.

“Kim is young and lacks experience, so there is a chance that he might develop an appetite for yet another risk or be tempted to engage in provocation to prove himself to the outside world,” the presidential adviser said.

Little is known about Kim Jong Un. For the first time, state media reported Tuesday on him observing military exercises with his father. The Korean Central News Agency also said in a report released early Thursday that he attended a concert with Kim Jong Il and other top party, state and military officials.

Under a 2007 deal, North Korea agreed to disable its main nuclear complex in Yongbyon north of Pyongyang in return for 1 million tons of fuel oil and other concessions and in June 2008 blew up the cooling tower. But the disablement came to halt as the North wrangled with Washington over how to verify its past atomic activities.

The Institute for Science and International Security said the satellite images showed heavy construction and excavation equipment and trucks at the Yongbyon site and construction of two small buildings near the site of the destroyed cooling tower.

“It is unclear if the activity seen in this image represents preparation for construction of a new cooling tower or preparation for construction of other buildings or structures for some other purpose,” ISIS said.

North Korean Vice Foreign Minister Pak Kil Yon vowed at the United Nations last week that the North would strengthen its nuclear capability in response to what he described as hostile moves by the United States.

North Korea is believed to have enough weaponized plutonium for at least a half dozen atomic bombs.

The North’s plutonium was extracted over a number of years from spent fuel rods from its Yongbyon reactor.

In Tokyo, U.S. Assistant Secretary of State for East Asian and Pacific Affairs Kurt Campbell called for “clear signs from the North Koreans that they are prepared to take the steps that they’ve committed themselves to in the past” regarding denuclearization. He spoke to reporters in Tokyo before leaving for South Korea, where he was to meet officials Thursday.

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Associated Press writers Kelly Olsen in Seoul and Malcolm Foster in Tokyo contributed to this report.