YONGBYON, North Korea (AP) — The gray cooling tower crumbled behind billowing dust clouds in seconds Friday, reducing the structure at North Korea’s nuclear reactor into a pile of rubble. It was a choreographed show by the communist regime meant to affirm an intention to stop making atomic bombs.

From a distance, smiling diplomats from the United States and other nations snapped photos of the blast that destroyed part of the heart of the North’s nuclear weapons program.

“As you all saw, the cooling tower is no longer there,” said Sung Kim, the U.S. State Department’s top expert on the Koreas who attended the demolition. “This is a very important step in the disablement process, and I think it puts us in a good position to move into the next phase.”

The 60-foot-tall cooling tower at the Yongbyon nuclear center had been the most visible symbol of the North’s nuclear program and a focus for U.S. satellite surveillance. Steam spewing from the tower meant that the North’s main nuclear reactor was operating to make plutonium.

Just before detonation, red warning flares were fired into the clear sky. At 5:10 p.m. (4:10 a.m. EDT), an explosion at the base of the tower sent it collapsing into a cloud of dust and smoke that blew over grassy fields along a small river.

After the explosion, the site was littered by broken columns of reinforced concrete and other shattered pieces of the tower shown in video of the site by international video news agency Associated Press Television News.

The tower’s destruction was not mentioned by the North’s media or shown on state TV broadcasts.

Ri Yong Ho, director of safeguards at North Korea’s Academy of Atomic Energy Research, was the most senior Pyongyang official present and shook hands with Kim after the blast.

“The demolition of the cooling tower is proof that the six-party talks have proceeded a step further,” Ri said, referring to the nuclear disarmament negotiations.

Blowing up the tower was intended to demonstrate North Korea’s commitment to forgo atomic weapons ambitions that culminated with its first nuclear test detonation in 2006.

Its destruction came in response to U.S. concessions announced Thursday to remove Pyongyang from terrorism and sanctions blacklists after the North delivered a long-awaited declaration of its nuclear programs.

North Korea praised Washington’s moves to lift sanctions but also urged the U.S. to completely abandon its “hostile policy” against the regime.

“The measure taken by the U.S. to lift the major sanctions … should lead to totally withdrawing its hostile policy toward the (North) in all fields in the future,” Pyongyang’s Foreign Ministry said in a statement, carried by the official Korean Central News Agency.

“Only then can the denuclearization process make smooth progress,” it said.

The tower, designed to carry off waste heat to the atmosphere, is a key part of the North’s five-megawatt atomic reactor. But its destruction carries little practical meaning because the plutonium-making reactor has already been largely disabled so it cannot be restarted easily.

Still, the demolition offered the most dramatic moment yet in the disarmament negotiations — involving North and South Korea, the U.S., China, Japan and Russia — that have dragged on for more than five years and suffered repeated deadlocks and delays.

“It is important to get North Korea out of the plutonium business, but that will not be the end of the story,” Secretary of State Condoleezza Rice said in Kyoto, Japan, on the sidelines of a meeting of the Group of Eight industrialized countries.

South Korean nuclear negotiator Kim Sook told reporters in Seoul that the disabling process would take several more months to complete, and that countries at the negotiations were discussing when to convene the next round of disarmament talks.

North Korea’s declaration does not address its alleged uranium enrichment program or suspicions of its nuclear proliferation to other countries, such as Syria.

The declaration, which was delivered six months later than the country promised and has not yet been released publicly, is said to only give the overall figure for how much plutonium was produced at Yongbyon — but no details of bombs that may have been made.

Experts believe North Korea has produced up to 110 pounds of weapons-grade plutonium, enough for as many as 10 nuclear bombs.