North Korea may have built a secret facility for producing weapons-grade plutonium, reports suggest.
There are reports that North Korea may have built a secret facility
US officials quoted in the New York Times say sensors on North Korea's borders have detected elevated levels of a gas produced as spent nuclear fuel is reprocessed into plutonium.
Because computer analysis appears to rule out the source of the gas as North Korea's main nuclear plant at Yongbyon, the levels of Krypton 85 imply the possible existence of another, hidden facility.
The reports came as UK Prime Minister Tony Blair had talks in South Korea with South Korean President Roh Moo-hyun, where they discussed the threat posed by North Korea's nuclear programme.
Both leaders agreed on the need for peaceful dialogue to resolve the crisis, and for North Korea to give up its nuclear weapons programme.
Mr Blair told reporters: "We want to resolve the issue of North Korea and its nuclear weapons programme and the export of nuclear weapons technology, by peaceful and constructive dialogue."
The South Korean president said the situation surrounding the standoff over the North's nuclear ambitions was more stable than six months ago and the views of the countries involved were converging.
On Friday, the head of the UN nuclear watchdog on Friday called Pyongyang "the most serious threat" to nuclear proliferation.
Mohamed ElBaradei, Director General of the International Atomic Energy Agency, expressed concern over the latest reports that North Korea was reprocessing fuel rods.
North Korea claimed last week that it had produced enough plutonium to start making nuclear bombs.
South Korea is watching the North ever more closely
The US Government said it did not discuss intelligence matters but that North Korea had no legitimate use for plutonium.
The raised levels of the gas provide the first evidence that a second plant may be operating in North Korea.
Speaking in Japan on Saturday, Mr Blair said that the Communist state posed a real danger that could not be ignored.
However, his East Asian tour is being overshadowed by the death of a UK Government scientist, David Kelly, at the centre of a row over Iraq's alleged weapons of mass destruction.
Mr Blair has called for restraint but the BBC Seoul correspondent says he will continue to be dogged by questions about the government's handling of the dispute.
Dr Kelly had been named by the government as the possible source for a controversial BBC story on Iraq.
US intelligence officials are said by the New York Times to be wary about making any final judgments about the new evidence in North Korea.
They are keenly aware that, as in Britain, assessments of Iraq's nuclear programme have sparked a national debate over whether intelligence was exaggerated, and have cast doubts on all the agency's findings, the paper says.
While on his brief visit to Seoul, Mr Blair will hold talks with South Korean President Roh Moo-hyun.
They will discuss the threat from North Korea as well as economic relations.
South Korean officials say they hope Mr Blair can use his influence on the world stage to encourage a diplomatic solution.
Mr Blair is backing the United States in its demand for negotiations involving all key players in the region.