By Yi Whan-woo

North Korea appears to be preparing for another nuclear test despite probable aftershocks from its sixth and largest nuclear test, Sept. 3, still occurring, according to American experts on Pyongyang.

In a joint analysis released Monday, Frank Pabian, Joseph Bermudez Jr. and Jack Liu, citing satellite imagery, said there has been a "consistently high level of activity" at North Korea's main nuclear test site in Punggye-ri.

The analysis was released by 38 North, a U.S. website specializing in North Korea-related issues.

The analysts claimed the suspected activity was detected at the West Portal, leading to an unused tunnel complex, but not the North Portal, where five of the six tests took place.

"Despite the continuation of small tremors near Mt. Mantap since North Korea's last nuclear test, tunnel work at the Punggye-ri Nuclear Test Site is still underway," they said. "These efforts continue to be concentrated at the West Portal, leaving the North Portal…mostly dormant and likely abandoned, at least for the time being. At the West Portal, there has been a consistently high level of activity since North Korea's last nuclear test."

The activity included the routine presence of vehicles and personnel around the portal, movement of mining carts to an adjacent spoils pile and signs of fresh spoils being dumped there.

These activities suggest that tunnel excavation is underway at the West Portal and that the reclusive state is expanding the site's "potential for future nuclear testing", according to the experts.

The three scholars said in their previous analysis, Nov. 6, that "significant movement of equipment, mining carts, material and netting" were detected around the West Portal.

The analysis came after the U.S. Geological Survey (USGS) said it detected two tectonic tremors of magnitude 2.9 and 2.4 near the Punggye-ri nuclear test site, Saturday.

The USGS said the tremors were probably aftershocks from the sixth nuclear test.

A series of quakes near Punggye-ri took place since the sixth nuclear test, triggering concerns that the test might have damaged the mountains and other surrounding areas.