North Korea seems to be working on making a "shortcut" to a tunnel at its nuclear test site in Punggye-ri, apparently aimed at making speedy preparations for a seventh underground nuclear experiment, according to government sources in South Korea on Sunday.

If carried out, it would be the secretive nation's first known nuclear test in 4 1/2 years.

South Korea's military and intelligence authorities have detected signs of the country focusing on restoring Tunnel 3 in the mountainous northeastern area, they said.

"(The North) abruptly stopped its initial construction work to restore the entrance to Tunnel 3, and it is digging up the side (of the tunnel)," a source said, requesting anonymity. "In this way, it seems like it will be possible to restore (the facilities) in a month."

Concerns have grown over the odds that the Kim Jong-un regime will push for another nuclear test since it test-fired a long-range missile last week, putting a formal end to its self-imposed moratorium on major strategic provocations.

Experts say the North may test a small tactical nuclear weapon that can be loaded on ballistic missiles around the key national holiday on April 25 to commemorate the founding of the Korean People's Revolutionary Army (KPRA).

There are reportedly four tunnels in the Punggye-ri zone, which was formally shut down in 2018, with demolition work carried out in front of a small group of invited foreign journalists.

Some observers said it would be difficult to immediately restore Tunnels 1 and 2, where the North carried out its previous nuclear tests, while Tunnels 3 and 4 could be usable after restoration work. (Yonhap)