North Korea has been restoring demolished tunnels at its only known nuclear test site in the country’s northeast, South Korea’s military announced on Friday, in the latest indication that Pyongyang may be preparing for a future underground nuclear weapons test.

South Korean and U.S. authorities are “closely monitoring related activities in close cooperation,” Seoul’s Ministry of National Defense said in a statement to reporters. 

The DPRK claimed to blow up the tunnels at Punggye-ri in May 2018, inviting international reporters to witness the demolition a month after North Korean leader Kim Jong