PYONGYANG, North Korea (CNN) -- North Korea on Friday destroyed a water cooling tower at a facility where officials acknowledge they extracted plutonium to build nuclear weapons, CNN's Chief International Correspondent Christiane Amanpour reported from the scene.

The cooling tower is demolished at the Yongbyon nuclear complex near Pyongyang, North Korea.

The massive implosion, which came at about 5pm local time Friday at the Yongbyon facility, was intended to be a powerful public symbol of a move to end nuclear activities by the Communist nation once branded a member of an "axis of evil" by U.S. President George W. Bush.

The destruction of the highly visible symbol of North Korea's long-secret nuclear program came just a day after the country released details of its program.

"They fired a warning flare and then in three minutes the whole thing came tumbling down in a massive cloud of smoke," Amanpour said. Watch Amanpour's report from the scene. »

"There was a moment of stunned silence as the magnitude of what had happened sunk in," Amanpour said.

U.S. State Department officials and observers from the International Atomic Energy Agency (IAEA) watched from a reviewing stand on a ridge about 1,000 yards away, she said.

"This is a very significant disablement step," the U.S. envoy to North Korea, Sung Kim, said.

Nuclear experts say that the plant's destroyed central water-cooling tower would take a year or longer to rebuild if North Korea were to try using the plant again.

"This is a critical piece of equipment for the nuclear reactor," said analyst John Wolfsthal, of the Center for Strategic and International Studies, who has been following North Korea since the 1980s. "Without this facility, the reactor can't operate and can't produce more plutonium for weapons." Watch the tower being demolished »

North Korea has been dismantling other parts of the facility under the watchful eyes of representatives of the five other nations, including the U.S., that have been involved in six-party talks aimed at ending Pyongyang's nuclear weapons program.

On Thursday, North Korean officials turned over to China a 60-page declaration, written in English, that details several rounds of plutonium production at the Yongbyon plant dating back to 1986. Watch President Bush claim progress over the N. Korea nuclear issue »

In it, North Korea acknowledges producing roughly 40 kilograms of enriched plutonium -- enough for about seven nuclear bombs, according to the U.S. State Department.

In response, Bush said he would lift some U.S. sanctions against North Korea and remove the country from the State Department's list of state sponsors of terrorism.

But he made clear that other sanctions remained in place on North Korea -- which has been on the terrorism list since its alleged involvement in the 1987 bombing of a South Korean airliner which killed 115 people.

"The United States has no illusions about the regime in Pyongyang," Bush said. "We remain deeply concerned about North Korea's human rights abuses, uranium enrichment activities, nuclear testing and proliferation, ballistic missile programs and the threat it continues to pose to South Korea and its neighbors."

U.S. analysts will pore over the document to resolve Washington's outstanding concerns, which include questions about the extent of North Korea's proliferation of nuclear technology and the status of any uranium enrichment program. Watch what's still unknown after Pyongyang's declaration »

Friday's event at the nuclear plant marked at least an effort by North Korea -- dubbed "The Hermit Kingdom" in the international community for its isolationist tendencies -- to show the world a good-faith effort to end its nuclear weapons program.

Media outlets from the five other nations involved in the talks were invited to view the tower's implosion, a rare move in North Korea. CNN was among the media outlets present.

But there appeared to be no mention of the declaration or planned implosion at Yongbyon on the Web site of North Korea's state-run news agency on Thursday. And an article Wednesday noted the 58th anniversary of the Korean war, calling it "a war of aggression started by the U.S. imperialists in an attempt to occupy the whole of Korea."

CNN's Chief International Correspondent Christiane Amanpour contributed to this story from Pyongyang.

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