Reports surfaced on Friday revealing that the North Korean regime is digging a new tunnel at its nuclear test site to conduct new nuclear weapon tests, according to South Korean media.
The nuclear site, which is located on North Korea's east coast and which has already seen three previous nuclear tests, has recently been filled with busy workers and vehicles digging out the new tunnel, revealed a government source quoted by Yonhap news agency as reported by Reuters.
"The fact that they are constructing a new tunnel indicates the intention is to conduct a nuclear test at some point," the unnamed source explained, adding that it had yet to be proven that the test would be held imminently.
Jeong Joon-Hee, a spokesperson for South Korea's Unification Ministry, declined to confirm the report, but said his nation and the US are closely monitoring Pyongyang's nuclear activity.
The report airs just days before a meeting in Seoul on Sunday, in which South Korean, Japanese and Chinese leaders will hold a summit that is to include a discussion of North Korea's nuclear arsenal.
US Defense Secretary Ashton Carter is also to visit South Korea on Sunday to discuss North Korea's nuclear threat.
North Korea's last nuclear test was held in 2013, leading to criticism even from its main supporter China.
UN Security Council resolutions ban ballistic trade with North Korea, and also ban it from launching rockets, given that the same "peaceful" space program technology is used to fire cataclysmic ballistic missiles. Nevertheless, the state has flouted those resolutions on numerous occasions.
It was revealed in May that North Korea was upgrading and expanding Sohae missile launch facility, allowing it to launch more sophisticated rockets. Earlier that month the regime successfully launched for the first time a submarine-launched ballistic missile (SLBM) capable of carrying a nuclear warhead from underwater. That missile joins a mobile ballistic nuclear missile unit it recently added to its arsenal.
Chinese experts warned the US in April that American estimates are wrong and North Korea actually has 20 nuclear weapons, with that arsenal to double next year thanks to the regime's higher than anticipated advanced enrichment capabilities.
The Obama administration has said that despite all this, it is willing to be as "flexible" with North Korea as Iran, calling for a new nuclear deal after the last one in the 1990s paved Pyongyang's way to the nuclear bomb.
North Korea has been sharing its technology with Iran, with reports of a delegation of North Korean nuclear experts visiting Iranian nuclear installations in April - after the framework agreement limiting Iranian nuclear activity had already been presented by Western powers negotiating with Tehran.
It was reported in April that US President Barack Obama hid intel from the UN about North Korea transferring rocket components needed to create a nuclear missile to Iran even during the nuclear talks, to try and prevent the UN from acting on the information with increased sanctions.