US technical experts arrived in North Korea on Tuesday to discuss disabling a nuclear programme which has already produced at least one atomic bomb.
The US team led by Sung Kim, State Department director for Korean affairs, crossed from South Korea at the border truce village of Panmunjom, the US military said.
It will join Russian and Chinese delegates in Pyongyang later on Tuesday to begin a five-day survey of key nuclear facilities.
The visit is seen as a hopeful sign that the communist state, which tested its first atomic bomb last October, is serious about permanently shutting down its nuclear plants.
Kim, who is also deputy US nuclear negotiator at the six-party talks on North Korean nuclear disarmament, held talks over dinner late Monday with his South Korean counterpart Lim Sung-Nam.
Lim said Kim was accompanied on his trip by six US nuclear experts and one each from Russia and China. South Korea and the United States agreed that steps to disable the facilities "must be taken in an efficient and very swift manner," he added.
The United States, China and Russia are the three nuclear-armed powers negotiating with North Korea in the talks which also include South Korea and Japan.
The experts will report back to the next session of six-party talks, expected this month.
In a landmark February agreement North Korea agreed to declare and disable all its nuclear programmes in return for aid, security guarantees and major diplomatic benefits.
In July it shut down its only operating reactor at the Yongbyon complex in return for 50,000 tons of fuel oil.
The International Atomic Energy Agency in August confirmed the shutdown, along with the closure of a nuclear fuel fabrication plant, a reprocessing plant and a separate 50 megawatt reactor, only partly built, at Yongbyon.
In addition, a 200 megawatt reactor under construction at Taechon was shut.
The next step is to permanently disable them by encasing them in concrete or some other method -- something the experts will advise on.
If the North declares and disables all its plants it will receive another 950,000 tons of fuel oil or equivalent energy aid. The accord also envisages the normalisation of relations with the United States and Japan, an end to US trade sanctions and a formal peace treaty on the Korean peninsula.
It does not specifically mention any existing nuclear weapons or plutonium stockpiles. The North has enough plutonium to build about five to 12 nuclear weapons, according to various estimates.