(CNN) -- 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 wouldn'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.