WASHINGTON (AP) ― A U.S. research institute said Thursday North Korea has accelerated excavation at a site used for underground nuclear test explosions, though a test doesn’t appear imminent.

The findings, based on satellite photographs, were released as Secretary of State John Kerry and his South Korean counterpart warned the North against any possible aggression.

The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said that the North likely started work last May on a new tunnel at the northeastern test site at Punggye-ri, where it has conducted its three previous nuclear explosions, the latest in February 2013. The institute estimates that the pile of earth excavated from it has doubled since the start of the year.

The findings are based on commercial satellite photographs, the latest taken Feb. 3.