There hasn't yet been any official North Korean reaction to the destruction of the most visible symbol of its nuclear program, but a U.S. diplomat who witnessed it said Saturday that the big blast saddened government officials there.

A day after North Korea blew up the cooling tower at its Yongbyon reactor complex to demonstrate its commitment to abandoning nuclear weapons, the State Department's top Koreas expert said he believed the event was an emotional loss for the Stalinist state.

"I detected ... a sense of sadness when the tower came down," said Sung Kim, who traveled to Yongbyon, about 60 miles north of the capital of Pyongyang to watch the demolition of the 60-foot-tall cylindrical structure.

"There is a significant degree of emotional attachment to the Yongbyon facilities," he told reporters in the South Korean capital after briefing Secretary of State Condoleezza Rice and other officials about the destruction of the tower on Friday.

The reclusive nation, one of the most isolated in the world, has yet to inform its citizens of the development, which came amid a flurry of activity in the international effort to get North Korea to give up atomic arms, and it is not clear if or when it will.

Kim said the sadness was most apparent on the face of Ri Yong Ho, the director of safeguards at North Korea's Academy of Atomic Energy Research who was the most senior Pyongyang official present, but was shared by other North Koreans who were there.

"(You could tell) just looking at the expression of the Yongbyon engineers who were on the site when this happened," said Kim, who shook hands with Ri after the smoke cleared and the cooling tower had vanished from the landscape.

"He said he just hoped that this would contribute to peace and stability," Kim said, adding that he understood the North Koreans had spent up to two weeks preparing the tower for demolition.

Wearing the same suit in which he witnessed the explosion, he said he had been struck by the power of the blast, which he watched with other invited guests from a hilltop about a kilometer away.

"It was quite dramatic, a huge blast and a complete collapse," Kim said, adding that it had taken almost two seconds to hear the explosion after the detonation and clouds of smoke enveloped the cooling tower.

The destruction of the tower marked the end of the first-phase of the denuclearization process in which the Yongbyon facility, where the North produced plutonium, was to be disabled.

It followed the presentation by North Korea on Thursday of a 60-page accounting of its nuclear activities that triggered an announcement from President Bush that he was moving to ease some sanctions on Pyongyang.

U.S. officials have stressed that the North Korean declaration must still be verified and that the cooling tower destruction is only one small part of the process. Kim said he believed that the North Koreans understood there was still much more to be done.

"I think they believe it's an important step but they also realize there's a lot more to do," he said. "There is no illusion about that."