A team of US technical experts has arrived in North Korea to discuss ways to disable the Communist state's nuclear programme.
Sung Kim leads the US delegation on a rare visit to North Korea
The seven-member delegation crossed from South Korea through the Panmunjom village in the Demilitarised Zone.
They will meet their Russian and Chinese counterparts and begin a four day survey of key nuclear facilities.
The visit by the foreign officials is another sign of the quickening diplomatic activity over the issue.
The experts plan to visit North Korea's main nuclear complex at Yongbyon, 100km (60 miles) north of the capital Pyongyang, to examine ways of disabling facilities there under a February international accord.
Announcing this latest mission on Friday, the US described it as "an ambitious phase".
At recent talks North Korean officials have apparently agreed a timetable for the completion of denuclearisation by the end of this year.
Despite conducting its first atomic weapons test last October, the North's only operating reactor has already been shut down in return for 50,000 tons of fuel aid.
But a far bigger carrot is being offered for the completion of the next step: substantial economic assistance, security guarantees and major diplomatic benefits for a permanent disablement of all nuclear facilities, BBC's John Sudworth reports from Seoul.
One method would be to encase them in concrete and this visit by the three nation team will help determine exactly what method to use.