After North Korea hinted at conducting a nuclear test in defiance of new U.N. sanctions against its rocket launch, South Korea has stepped up monitoring of tunnels at the communist state's test site in its northeast, military officials said Wednesday.

Citing satellite images that revealed excavation and related movement at the Punggye-ri test site, Seoul intelligence officials said Pyongyang is ready to detonate a nuclear device on a few days' notice, but the decision to go ahead with the test will be a political one.

The North had repaired extensive rain damage at the sprawling facility with three known tunnel entrances and multiple support buildings in mountainous terrain in the nation's northeast, they noted.

"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.

The latest move comes as the communist state earlier in the day condemned a new round of U.N. sanctions and hinted it may carry out a nuclear test. The UN Security Council unanimously passed new sanctions against North Korea for the Dec. 12 rocket launch and warned "significant action" for another atomic test.

The North conducted tests on the east and west sides of the shafts in 2006 and 2009, respectively, following long-range missile launches. The eastern shaft is known to have been shut down, officials said.

 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 U.N. resolution condemned it as a disguised ballistic missile test which violated current sanctions imposed after the North's nuclear tests in 2006 and 2009. (Yonhap News)