"Recent commercial satellite imagery has identified two new tunnel entrances and continued excavation at the Punggye-ri nuclear test site," Johns Hopkins University researchers wrote on their 38 North blog Wednesday.
North Korea has conducted three nuclear tests since 2006. Two of them were carried out at the Punggye-ri test site.
The researchers said the tunnels may be for preparation for another nuclear nest, to accommodate future traffic flow or for ventilation.
"There are no signs that Pyongyang plans to conduct a nuclear test in the immediate future," they said. "However, these ongoing activities as well as upgrades to the site's support areas indicate North Korea is preparing to conduct additional detonations in the future as part of its nuclear weapons development program."
The Johns Hopkins team said it takes more than a year to complete the operations it observed at Punggye-ri so caution and additional monitoring is warranted.