"Earth is being currently extracted from a tunnel where the previous nuclear test was carried out. North Korea is thoroughly preparing for the next nuclear test," Kono said, as quoted by the Kyodo news agency.
The statement comes amid the ongoing thaw in relations between the two Koreas, which started before the Winter Olympics, held in South Korea’s Pyeongchang last month, with North Korean athletes participating in the Games and the sides exchanging high-level delegations.
South Korean President Moon Jae-in and the North Korean leader have agreed to hold a summit on April 27, ahead of an expected meeting between US President Donald Trump and Kim in late May.
Moon also voiced an idea to hold a trilateral summit to discuss the issues of denuclearizing the Korean Peninsula following last year’s nuclear test and ballistic missile launches carried out by North Korea.