North Korea has demolished the cooling tower at its Yongbyon nuclear reactor, in a symbol of its commitment to talks on ending its nuclear programme.
International television crews were invited to witness the tower being blown up.
The move comes a day after the isolated state handed over long-awaited details of its programme, but no account of the weapons many fear it has stockpiled.
In return, the US has agreed to lift some of its economic sanctions.
State media reported North Korea's Foreign Ministry had welcomed the US move on sanctions, regarding it as a "positive step".
The Yongbyon reactor was shut in July last year as part of a six-party agreement reached 16 months ago, when the North said it would scrap its nuclear ambitions in return for aid and diplomatic concessions.
YONGBYON COOLING TOWER
It was 20m (65ft) high
In operation since March 2003
A key element of the reactor, although Yongbyon was already in the process of being decommissioned
Cooling tower is a simple piece of equipment that could easily, though not invisibly, be rebuilt
Analysts say that while the destruction of the tower is not, in itself, a huge step forward, it is still being seen as an important gesture.
"It was a significant and very important step," said US state department official Sung Kim, who witnessed the event.
"As I saw it, it was a complete demolition."
The US has agreed to scrap sanctions under the Trading with the Enemy Act, and will begin the process of removing the state from its list of terrorist sponsors in August - but only if North Korea's nuclear declarations are verified.
"We assess this as a positive step and welcome it," a foreign ministry spokesman told the North's official Korean Central News Agency.
The reactor, 96 km (60 miles) north of Pyongyang, produced plutonium for the North's nuclear weapons test in 2006.
The BBC's world affairs correspondent said that blowing up the cooling tower meant it would take North Korea about a year to revive its plutonium production, and it would be obvious if it was doing so.
However, he added that "bigger hurdles remain" - including the critical issue of actual weapons stockpiles, as well as suspected North Korean proliferation activities - particularly the supposed Syrian connection.
South Korea said on Friday it hoped a new round of six-party talks - which also include North Korea, Japan, Russia, China and the US - could begin next month.
Meanwhile, the negotiating team has been poring over Pyongyang's 60-page declaration, submitted on Thursday.
Six months overdue, the account is expected to cover the North's plutonium production activities.
But the dossier omits any tally of its nuclear arsenal, any mention of a suspected uranium enrichment programme or claims it helped Syria build a nuclear facility, all denied by Pyongyang.
Mr Bush has emphasised moves to take the North off the US terror list would not begin for 45 days, and only if its nuclear declaration was verified.
But former US envoy to the UN John Bolton labelled the decision "shameful" and the "final collapse of Bush's foreign policy".
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