North Korea on Friday blew up the cooling tower of its atomic reactor at Yongbyon to symbolise its commitment to nuclear disarmament, one day after it handed over details of its atomic programmes.
TV footage showed the 30-metre (99 foot) tower engulfed in a huge cloud of smoke as a landmark piece of the country's nuclear history collapsed in ruins.
"It was a significant and very important step," US State Department official Sung Kim told a reporter at the scene.
"As I saw it, it was a complete demolition."
The cooling tower at Yongbyon, 96 kilometres (60 miles) north of Pyongyang, was the most visible symbol of the communist state's decades-old pursuit of nuclear weapons.
It produced the plutonium for a programme which culminated in a nuclear test in October 2006.
The demolition was of symbolic value only since Yongbyon has already been shut down under a six-party disarmament pact.
But it came a day after the North handed over a long-awaited declaration of its nuclear activities, a move expected to end the stalemate in the six-party negotiations on disarming the poor and isolated state.
US President George W. Bush announced he was partially lifting some Trading With The Enemy Act sanctions in response.
He also notified Congress he was removing North Korea from the US list of state sponsors of terrorism, effective after a 45-day review period.
North Korea, in its first comment on the dramatic developments of the past two days, said it welcomed the US move on the terror list as a "positive step."
Bush in comments Thursday termed the declaration an "important step" but said it was only the start of the process.
"We will trust you only to the extent that you fulfil your promises," he told the country he once branded as part of an "axis of evil."
A battery of other sanctions against the North remain in force.
The delisting can also be put on hold if the North fails to meet US demands that it allow its declaration -- especially the size of the plutonium stockpile -- to be rigorously verified .
The declaration delivered to six party talks host China was said to give details of nuclear facilities and of Yongbyon's production over the past two decades of plutonium for bomb-making.
It does not disclose any information on nuclear weapons, an issue to be tackled in the next phase of the six-party deal.
China on Friday handed over the dossier to the other six-party nations, said chief US negotiator Christopher Hill.
"All the nuclear materials and fissile materials and bomb-making materials is listed," Hill told reporters in the Japanese city of Kyoto, where Group of Eight foreign ministers were meeting.
The ministers, including US Secretary of State Condoleezza Rice, said the goal should be the "abandonment of all nuclear weapons and existing nuclear programmes by North Korea."
US National Security Adviser Stephen Hadley said the declaration did not answer US allegations of nuclear proliferation to Syria, or its claims of a past secret enriched uranium weapons programme.
"We're in a situation of not quite admitting, not denying, but opening the door for us to be able to try and get greater clarity," Hadley said.
Rice said North Korea must grant access to the reactor core and radioactive nuclear waste. But she called the 60-page document a "good step forward."
The North is getting one million tons of fuel oil or equivalent energy aid, plus the terrorism delisting, in return for disabling Yongbyon and delivering the declaration.
The six parties -- North and South Korea, Japan, Russia, China and the United States -- are expected to meet in Beijing early next month.
They will discuss ways to verify the document, complete the disablement and prepare for the final phase: the dismantlement of plants and handover of all nuclear material including weapons.
