"Considering various possibilities, (we) are closely tracking and monitoring (the North's move) in cooperation with the United States," Army. Col. Roh Jae-cheon, a spokesman for the JCS, told reporters.

The South's military does not rule out the possibility of the North pushing for a "different type of nuclear test from the past," he added without elaborating.

The North's military, he said, seems to be continuing its annual winter training but shows no "unusual" indications.

A series of reports suggest that Pyongyang is edging closer to detonating a bomb at its Punggye-ri nuclear site.

In particular, fresh satellite imagery of the northeastern area shows up to 100 people gathered near the key facilities, according to the 38 North website carrying news on North Korea.

It adds to the website's earlier reports of vehicles and communication equipment recently spotted at the site.

North Korea watchers raise the possibility that the North will carry out a "Pakistan-type" test. Pakistan blasted six nuclear bombs all using different materials on May 28 and 30 in 1999 in an apparent to bid to collect data as comprehensive as possible.

The North is also known to have a number of tunnels at the Punggy-ri site, which enable it to test several devices almost simultaneously.

The sixth nuclear explosion by the North, if conducted, would aim to master the technology to fit a nuclear warhead on to a missile and maximize an explosive yield, experts pointed out.

"North Korea claims that it has already miniaturized a nuclear bomb. Thus, there is a chance that it will test a boosted fission bomb," Lee Chun-geun, a researcher at the Science and Technology Policy Institute, said. "It could be 150-200 kilotons."

The force of each explosion in its previous tests in 2006, 2009, 2013 and twice last year reportedly stayed below 10 kilotons.

Chances are high that the North will focus on testing a "boosted fission" weapon in the run-up to developing a hydrogen bomb, according to observers.

On the timing of another nuclear test, they said it's possible within the coming weeks, given Pyongyang's record of using the provocation to commemorate major political events and draw international attention.

US President Donald Trump and his Chinese counterpart Xi Jinping plan to hold summit talks in early April, with the 105th birth anniversary of the North's founder Kim Il-sung to follow on April 15.

The Presidium of the Supreme People's Assembly, which is the nation's rubber-stamp parliament, also plans to hold a main session on April 11 ahead of the 85th anniversary of the founding of the Korean People's Army on April 25. (Yonhap)