South Korean officials said Sunday that the North may conduct an additional nuclear test as there has been a sharp increase of activity in North Korea’s Punggye-ri nuclear test site, Yonhap reported Sunday. The report comes as the Kim Jong un regime said Saturday that the nuclear weapons being developed were an “inevitable” self-defense.

Several government sources said: “Compared to last month, the frequency of vehicle, workforce and equipment movements increased two- to threefold recently” at the Punggye-ri nuclear test site in the country’s northeast. “Related officials concluded that it is a convincing sign that North Korea is preparing for its fifth nuclear test, and they are keeping close tabs on the situation,” the Yonhap report quoted a source as saying. The source also said that vehicles were seen moving in and out of the North Portal tunnel, adding that the vehicles could be carrying nuclear technicians.

One of the sources said that since the beginning of April, the movement of vehicles and people at the facility has increased. The source added, according to on Yonhap: “If they are signs of nuclear test preparations, it seems (the preparations) are in the final stages.”

Concerns have been growing over the possibility that the Kim Jong Un regime is continuing additional provocations like nuclear detonation tests while Pongyang prepares to hold its first congress of the Ruling Workers Party in early May, after over 30 years. The country has also claimed to have secured intercontinental ballistic missile technologies like re-entry and engine technologies, despite harsh sanctions from the United Nations Security Council.

Pyongyang, which conducted its fourth nuclear test in January and a rocket launch in February, has also threatened to attack South Korea and the U.S. over their ongoing annual military drills. Seoul has been closely monitoring the nuclear site to find out if Pyongyang is preparing to test a miniaturized nuclear warhead.

“If North Korea goes ahead with a fifth nuclear test, it may announce (after the test) a success in testing a miniaturized nuclear warhead that could be loaded onto an ICBM,” another source said, according to Yonhap.

Pyongyang has called the joint military drills a possible cover for a U.S.-led invasion, but Washington and Seoul have claimed that the drills are purely defensive. In its latest statement, cited in another Yonhap report Sunday, North Korea’s KCNA said: “The DPRK's (Democratic People's Republic of Korea) access to nuclear weapons is not a threat but an inevitable self-defensive option for protecting the country and nation from the nuclear disaster to be brought by the U.S.,” adding: “The stronger the striking capability of the DPRK's nuclear weapons grows, the more powerful the capability to deter aggression and war will become.”

Pyongyang also said: “The U.S. nuclear threat and blackmail and joint military drills are the source of pushing the situation on the Korean Peninsula to the brink of war,” adding: “It has posed a ceaseless nuclear threat to the DPRK, deploying nuclear weapons in South Korea and staging frantic nuclear war drills against the DPRK for over half a century.”

On Friday, the North conducted a failed test of a Musudan intermediate-range missile. The international community condemned the test while the U.N. called it a clear violation of resolutions.