United States government officials have said that North Korea has admitted it has a nuclear weapons programme.
The South Korean President, Kim Dae-jung, described the matter as "grave" but said he believed North Korea wanted to solve the issue through dialogue.
Japan has called on North Korea to "erase nuclear suspicions honestly".
North Korea is one of three states dubbed part of an "axis of evil" by US President George W Bush, along with Iran and Iraq.
US officials say the Bush administration is now consulting its allies and Congress before deciding what to do in light of the revelation.
North Korea's pursuit of nuclear energy began in the late 1950s when it signed protocols with its allies to secure nuclear technology and equipment, notably the former Soviet Union.
Despite numerous denials by Pyongyang, there had been suspicions that the reactors built in the decades that followed would be used for weapons production and might be capable of producing up to 8kg of plutonium annually.
In January 1992, North Korea joined the Non-Proliferation Treaty (NPT). But it rejected inspections of suspected nuclear fuel storage sites and threatened to withdraw from the NPT in 1993, triggering a nuclear crisis on the Korean peninsula.
Tensions reached their peak in June 1993 when US president at the time, Bill Clinton, warned that North Korea would be annihilated if it used nuclear weapons.
The two-year-long standoff was headed off in 1994 when Pyongyang agreed to freeze its nuclear programme in return for the construction of safe nuclear reactors for the impoverished country.
