SEOUL - South Korean and US intelligence sources on Sunday (July 17) said they have detected increased activity at a North Korean nuclear test site and have beefed up surveillance, Yonhap news agency reported.
The allies said activity at the site in the north-eastern region of Punggye-ri had been stepped up after Seoul and Washington on July 8 announced they planned to deploy the Terminal High Altitude Area Defense (Thaad) anti-missile system in South Korea.
Pyongyang last Monday threatened to take "physical action" over the deployment.
"There has been a noticeable increase in truck and personnel movement around the site in Kilju county," an insider, who wished to remain anonymous, was quoted as saying by Yonhap.
"There has been suspicious movement in the area since the Thaad announcement and Seoul is examining all angles of the change, including the possibility of Pyongyang moving to detonate another nuclear device," he said, without going into too much detail.
Another official was cited by Yonhap as saying that while the pick-up in traffic may be related to the North conducting routine maintenance work, it could be a sign that the reclusive country may be moving to carry out another nuclear test, the second this year.
Tensions are high since North Korea carried out its nuclear test in January, followed by a series of missile launches.
It detonated its first device in 2006, followed by two others in 2009 and 2013.