"An increased movement of people and cars has been spotted at the nuclear site. North Korea appears to be in the process of digging another tunnel," the official told the South Korean Yonhap news agency on the condition of anonymity.
In 2003, Pyongyang joined negotiations on the denuclearization of the Korean peninsula with the United States, Russia, China, Japan and South Korea, after it withdrew from the Nuclear Non-Proliferation Treaty that it ratified in 1985.
North Korea then withdrew from the talks, declared itself a nuclear power in 2005, and conducted nuclear tests in 2006, 2009 and 2013. Pyongyang has never made public how many nuclear weapons it possesses.
Pyongyang’s nuclear activities have received international condemnation.
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