"North Korea's Punggye-ri Nuclear Test Site, where North Korea has conducted six acknowledged underground detonations is still, as far as we can tell, fully operational," the website said.

It appeared that North Korea had abandoned one area of the site -- the North Portal -- after its sixth nuclear test in September, but "significant new tunneling" was spotted at the West Portal through early March, it said.

"That renewed tunneling was curtailed by mid-March, but not entirely stopped through early April, suggesting that either the tunnel was complete and ready for future renewed testing or that the slowdown simply mirrored the ongoing political changes underway," according to 38 North.

North Korean leader Kim Jong-un met with South Korean officials in early March and expressed his commitment to denuclearize. He invited US President Donald Trump to a summit where the issue would be on the table.

Trump accepted the offer on the spot, and the meeting is expected to take place in May or early June.

38 North said there is another potential test tunnel, accessible via the South Portal, that remains suitable for underground nuclear tests.

"In short, there is no basis to conclude that the Punggye-ri nuclear test site is no longer viable for future nuclear testing," according to the website. "There remain two portal areas located in more pristine competent rock that can be used for future tests if Pyongyang were to give the order. Whether that will stay an option will depend on reaching verifiable agreements that build on Pyongyang's pledge to shut down the facility." (Yonhap)