South Korea has detected no signs of North Korea trying to restore underground tunnels at its purportedly demolished Punggye-ri nuclear test site despite indications of maintenance work there, a Seoul official said Tuesday.

The official's assessment came after Olli Heinonen, a former deputy director-general at the International Atomic Energy Agency, told the Voice of America (VOA) earlier this week that the North is maintaining the site as evidenced by "trails of the cars and cleaning of snow."

In May 2018, the North claimed to have demolished the site in a show of willingness to denuclearize. The following month, it committed to work toward the "complete denuclearization" of the Korean Peninsula at the first-ever summit with the United States in Singapore.

"We have identified the reported maintenance activities at part of the facilities there, but we have not detected any signs of activities to restore tunnels," the official told reporters on condition of anonymity.

Commenting on the VOA report, the Joint Chiefs of Staff (JCS) said that the intelligence authorities of South Korea and the United States are keeping close tabs on North Korean activities at the site.

"Since the closure of the nuclear testing site in Punggye-ri, we have been paying close attention to related activities there," a JCS official told reporters on condition of anonymity. "To date, there is no noteworthy change."

In the VOA report, Heinonen cited two possible reasons for the North's activities to maintain the site, including the monitoring of any release of radioactivity from it.

"The other function is that they probably maintain the site so that they make a decision at a later date to still do tests," he said. "They might use some tunnels, which were not destroyed in 2018. So, I think it's kind of dual purpose at this point of time."

The signs of activities at the Punggye-ri site came after Pyongyang issued an apparent threat to discard its self-imposed moratorium on nuclear and long-range missile tests amid a protracted impasse in nuclear negotiations with Washington. (Yonhap)