North Korea has accelerated digging work at a site used for underground nuclear test explosions, a US research institute has said.
The findings, based on satellite photographs, were released as U.S. Secretary of State John Kerry and his South Korean counterpart warned the North against any possible aggression.
According to the Japan Times, the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies said that the North likely started work in May 2013 on a new tunnel at the northeastern test site at Punggye-ri.
The North has previously used the site to conduct three previous nuclear explosions, the latest in February 2013.
The institute estimated that the pile of earth excavated from it has doubled since the start of the year.
The new imagery from Punggye-ri is the latest sign that North Korea is moving in the other direction.
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38 North estimates that the amount of earth heaped near the new tunnel suggested a tunnel of around 1,000 meters long has been dug into the mountainside.
38 North said that North Korea probably has two other tunnels already complete at the site.