North Korea has completed all of the preliminary steps required to conduct a nuclear test , a South Korean government official said . The South detected the closure of the entrance of a tunnel at the Punggye-ri site in a northeastern region of North Korea . It means that Pyongyang is now ready for what could be its fourth nuclear test , the official said . `` This is the final step in preparing to test a nuclear device , '' the official said Thursday . `` In theory , there are seven to 14 days to conduct a test once the entrance is sealed . '' The official added that it was highly likely the North would test an enriched uranium device , as it does not seem to possess a lot of plutonium . The official also said the North might launch a long-range ballistic missile instead , if it decided not to conduct a nuclear test . This would `` show off their two-way track capability , '' the official said . Asked if the North had been able to construct a nuclear weapon small enough to fit on a missile , the official said : `` We believe that North Korea has made a considerable progress in technology to miniaturize the nuclear warhead and that they are really doing their best . In fact , North Korea is spending a substantial amount of money and time ( on this ) . '' The comments came the day before U.S. President Barack Obama arrived in South Korea as part of a visit to several Asian countries . Obama was previously in Japan and continues on to Malaysia on Saturday . In an interview with a South Korean newspaper ahead of his arrival Friday , Obama warned Pyongyang over the possibility of carrying out a new test . `` If North Korea were to make the mistake of engaging in another nuclear test , it should expect a firm response from the international community , '' Obama told Joonang Ilbo . The North Korean Foreign Ministry earlier this week described Obama 's trip as `` a reactionary and dangerous one as it is aimed to escalate confrontation and bring dark clouds of a nuclear arms race to hang over this unstable region . '' South Korea 's Defense Ministry said this week it had increased its military preparedness since Monday morning and was monitoring around the clock for signs of activity at the Punggye-ri site . North Korea announced last month that it would n't rule `` a new form of a nuclear test '' to strengthen its nuclear deterrent capabilities . Experts have speculated that that could refer to the testing of a uranium bomb . The reclusive regime in Pyongyang is known to have conducted three previous tests , all of them believed to be based on plutonium . The most recent one took place February 2013 . The past tests prompted international condemnation and the imposition of sanctions aimed at hampering the North 's weapons program . Since the last underground detonation , South Korean officials have repeatedly said that they believe North Korea is ready to carry out another test at any time , pending a political decision to go ahead with it .
South Korea official : South Korea ready for nuclear test . Official : Nuclear closure is last time to ready for test . Details come as U.S. President Obama visits South Korea .