“It is highly likely that site is capable of supporting additional tests at any time,” it said.

The satellite imagery from March shows that the North Portal, where the January test took place, remains active with footpaths around the buildings and what appears to be water draining from the entrance along the drainage ditch, 38 North said.

That may support the hypothesis that branching tunnels lead to other test chambers since the shaft used for the January test should have been completely sealed to prevent radioactivity leakage.

“If there are other unused shafts at the current North Portal, they are being drained in order to maintain them for future detonations,” it said.

The main support area also appears to be active with numerous vehicle tracks and footpaths visible, it said.

At the South Portal, snow has been cleared in the vicinity of the entrances to the tunnels, which were excavated in 2009 and 2012, indicating that they are being at least maintained for future tests, 38 North said.

North Korean leader Kim Jong-un threatened earlier this week that a new “nuclear warhead explosion test” and ballistic missile tests will be conducted “in a short time” to further bolster its nuclear missile capabilities.

Analysts say that the North is expected to continue threats and provocations as Kim tries to assert his leadership in the lead-up to a key Workers’ Party Congress in May.