Work appears to be continuing at North Korea’s nuclear site, Seoul’s military officials said Sunday, prompting speculation that the communist state is counting down to a fresh underground blast to celebrate the birthday of its late founding father and a forthcoming party congress.

The South Korean military has in recent weeks been bolstering surveillance as they had detected a sharp rise in the movement of labor, vehicles and other equipment in and out of the Punggye nuclear site in the country’s northeast. 

But the latest discovery does not necessarily point to any imminent explosion, a military official here said, adding that it was in line with a trend that has been identified around the area since North Korean leader Kim Jong-un called for a test of warheads and ballistic missiles to bring about “further increase in the reliability of our nuclear strike capabilities.”

“There have constantly been similar signs in recent weeks, and as we’ve repeatedly said that the North is ready to conduct another test at any time if needed, it is to be seen when it will actually take place,” the official said, referring to Defense Ministry spokesman Moon Sang-gyun.

Last Wednesday, the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies also reported that given commercial satellite imagery, a fifth experiment could not be ruled out, though it bore little evidence to say that it may take place “in the next few days.”

Photos taken on April 9 showed the presence of a vehicle at the North Portal, where the Jan. 6 fourth nuclear test was carried out, on top of a few other small vehicles, a truck and shipping crates found on April 11 near the main support area, the think tank wrote on its 38 North blog.

Concerns have been growing that the Kim Jong-un regime could press ahead with an additional provocation to mark the April 15 birthday of Kim Il-sung, the country’s founding leader and the incumbent’s grandfather, or the ruling Workers’ Party’s first congress in 36 years, anticipated to take place in early May.

State media on Saturday reaffirmed Pyongyang’s commitment to its nuclear program, calling it an “inevitable self-defensive option” to defend the threats posed by U.S. nuclear weapons and joint military drills with South Korea.

“The stronger the striking capability of the DPRK’s (North Korea’s) nuclear weapons grows, the more powerful the capability to deter aggression and war will become,” the official Korean Central News Agency said.

On Friday, the North launched its Musudan intermediate-range ballistic missile off the east coast, the South’s Joint Chiefs of Staff said. But it appeared to have exploded midair, vanishing off radar.

The missile, tested for the first time though projected to have been on deployment since the mid-2000s, is capable of reaching all over Japan and as far as Guam, with a range of up to 4,000 kilometers. The other Musudan that was delivered to the Wonsan area could also be fired in the coming days, military officials said.

The U.N. Security Council issued a press statement Friday condemning the failed move as a clear violation of resolutions banning the country from using any ballistic missile technology. It warned Pyongyang against another provocation, saying it would take “further significant measures in line with previously expressed determination.”

By Shin Hyon-hee (heeshin@heraldcorp.com)