South Korea has recently detected a sharp increase in vehicle and human activities at North Korea's Punggye-ri nuclear test site and concluded the country is likely to carry out an additional nuclear test before early May, officials said Sunday.

"Compared to last month, the frequency of vehicle, workforce and equipment movements increased two- to threefold recently" at the nuclear test site in the country's northeast, multiple government sources said.

"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 sources said.

They said vehicles were seen moving in and out of the site's North Portal tunnel, and they may be carrying nuclear technicians.

One of the sources indicated that since the start of April, there has been a growing increase in the movements of cars and humans, adding that, "If they are signs of nuclear test preparations, it seems (the preparations) are in the final stages."

Concerns have been growing in recent weeks that Pyongyang could undertake additional military provocations like a nuclear detonation test as the country will hold its first congress of the ruling Workers' Party of Korea in more than 30 years in early May.

In a series of recent warlike rhetoric, North Korea has also claimed to have secured key intercontinental ballistic missile technologies like the re-entry and engine technologies and threatened to conduct a "nuclear warhead detonation" test.

South Korea's officials are now closely monitoring the nuclear site to detect if North Korea is preparing to test a miniaturized nuclear warhead as the country has claimed.

"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.

In a related development, the U.S.-based website 38 North said last week that recent satellite imagery strongly indicates that North Korea has already begun or is set to start reprocessing spent nuclear fuel at its Yongbyon reactor site to harvest plutonium for nuclear weapons.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests: in 2006, 2009 and 2013. The North conducted its fourth nuclear test on Jan. 6, claiming it successfully detonated a hydrogen bomb.

South Korea has been certain that North Korea is technically ready to conduct a nuclear test at any time and the test would take place as soon as North Korean leader Kim Jong-un makes a decision.

South Korea predicts the test will likely come before the party congress.

The military provocation may further escalate tension on the Korean Peninsula following the North's fourth nuclear test and a long-range rocket launch earlier this year. (Yonhap)