YONGBYON, North Korea (AP) — North Korea destroyed the most visible symbol of its nuclear weapons program today, blasting apart the cooling tower at its main atomic reactor in a sign of its commitment to stop making plutonium for atomic bombs.

An explosion at the base of the cylindrical structure sent the tower collapsing into debris and dust that billowed into blue skies at 5:10 p.m. local time as journalists and diplomats looked on, according to footage filmed at the site by the international video news agency, The Associated Press Television News.

The demolition of the 60-foot-tall cooling tower at the North’s main reactor complex is a response to U.S. concessions after the North delivered a declaration Thursday of its nuclear programs to be dismantled.

“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,” said Sung Kim, the U.S. State Department’s top expert on the Koreas who attended the demolition.

After the tower’s tumble to the ground, Kim shook hands with Ri Yong Ho, director of safeguards at North Korea’s Academy of Atomic Energy Research, who was the most senior Pyongyang official present.

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