SEOUL, Jan. 23 (Yonhap) -- North Korea has completed all technical preparations for a nuclear test and can carry it out in a few days if it makes a decision, a South Korean intelligence source said Wednesday.
North Korea had dug up a tunnel for a test at its Punggye-ri nuclear test site, but the tunnel has now been plugged with dirt and concrete, the source said, suggesting that all measuring and other equipment has already been installed inside.
It was unclear when the tunnel was sealed.
"North Korea has completed technical preparations for a nuclear test," the source said. "If (North Korean leader) Kim Jong-un makes a political decision, the North can conduct a nuclear test in a few days."
Earlier in the day, North Korea hinted at the possibility of conducting a nuclear test after the U.N. Security Council adopted a new resolution condemning the country's Dec. 12 long-range rocket launch.
South Korean officials said they have stepped up monitoring of the test site.
North Korea had detonated nuclear devices at the Punggye-ri test site in 2006 and 2009, following long-range rocket launches.
South Korean officials earlier had said that the North had repaired extensive rain damage at the sprawling nuclear test site known to have three tunnel entrances and multiple support buildings.
"North Korea has continuously conducted computer simulations for an additional atomic test with data acquired from the first and second nuclear tests," a senior official said, asking for anonymity as he is not allowed to disclose information to media.
Seoul officials say the North is expected to detonate a nuclear device made of highly enriched uranium, which is difficult to immediately detect with available techniques, including airborne radioactivity, seismological and airborne sound wave tests.
"It seems that the North needs to develop highly enriched uranium to secure additional nuclear materials because of its limited quantity of plutonium," a senior intelligence official said, asking to be unnamed.
The North is capable of producing about 40 kilograms of HEU a year, intelligence officials said, considering North Korean officials at Yongbyon nuclear complex told U.S. nuclear scientist Siegfried Hecker, former chief of the Los Alamos National Laboratory, that 2,000 centrifuges were operational during his 2010 visit.
If that's correct, the North can produce up to two nuclear devices per year with that amount, they noted.
The defense ministry estimated in December that the North had spent about US$1.1-15 billion on its nuclear program.
Although Pyongyang insists the Dec. 12 rocket launch was aimed at sending an observation satellite into space, the UN resolution condemned it as a disguised ballistic missile test which violated current sanctions imposed after the North's nuclear tests in 2006 and 2009.