Presidential spokesman Yoon Young-chan said the closure of the Punggye-ri site would be done in public and foreign experts from South Korea and the US would be invited to watch.
According to scientists, the nuclear site may have partially collapsed in September.
On April 27, North Korean leader Kim Jong-un and South Korean President Moon Jae-in agreed to work to rid the Korean peninsula of nuclear weapons.
Their summit came after months of warlike rhetoric from North Korea.
On April 28, President Donald Trump said he would like to hold talks with Kim Jong-un “over the next three or four weeks” about the denuclearization of the peninsula.
President Moon Jae-in’s spokesman said that Kim Jong-un had stated he “would carry out the closing of the nuclear test site in May”.
Yoon Young-chan added that the North Korean leader had also said he “would soon invite experts of South Korea and the US to disclose the process to the international community with transparency”.
President Moon Jae-in’s office also said North Korea would change its time zone – currently half an hour different – to match that of South Korea.
North Korea has so far made no public comments on the issue.
Situated in mountainous terrain in the north-east, the Punggye-ri site is thought to be North Korea’s main nuclear facility.
The nuclear tests have taken place in a system of tunnels dug below Mount Mantap, near the Punggye-ri site.
Six nuclear tests have been carried out there since 2006.
After the last nuclear test, in September 2017, a series of aftershocks hit the site, which seismologists believe collapsed part of the mountain’s interior.
According to South Korea’s Yonhap news agency Kim Jong-un made an apparent reference to these reports, saying: “Some say that we are terminating facilities that are not functioning, but you will see that they are in good condition.”
The information about the Punggye-ri site has been gathered mainly from satellite imagery and tracking the movement of equipment at the location.
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