The threat posed by North Korea's nuclear program has reached an "extremely dangerous level," an adviser to South Korea's president said in comments published Wednesday.

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

The South Korean security official's comments were quoted Wednesday in the JoongAng Ilbo newspaper. Kim confirmed to The Associated Press that he made the comments Tuesday at a forum on Northeast Asia.

"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.

Pyongyang, however, pulled out of the so-called six-party talks, last year amid an international row over a suspected long-range missile test that North Korea said was a satellite launch.

"Should North Korea miniaturize 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, 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 facilities in different parts of the country.

He declined to elaborate on his reported comments.

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 even blew up its cooling tower. But disablement came to halt as Pyongyang 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 Korea is believed to have enough weaponized plutonium for at least half a dozen atomic bombs.

The North's plutonium, which is not a naturally occurring material, was extracted over a number of years from spent fuel rods at its Yongbyon reactor.