South Korean and U.S. intelligence sources on Sunday said they have detected increased activity at the North Korean nuclear test site in Punggye-ri and have beefed up surveillance.

The allies said there has been an increase in activity particularly after Seoul and Washington on July 8 announced they planned to deploy the Terminal High Altitude Area Defense system in South Korea.

"There has been a noticeable increase in truck and personnel movement around the site in Kilju county," said an insider, who wished to remain anonymous.

South Korean officials said that when 38 North, a U.S.-based website monitoring the communist nation, reported early last week that there was more activity at Punggye-ri in the northwestern part of North Korea, most watchers did not pay too much attention, but they are starting to notice now.

"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.

After the announcement of the decision to deploy the anti-ballistic missile system was made, the North's General Staff of the Korean People's Army warned that Pyongyang will take "physical action" to respond to what it called provocations by hostile forces.

Related to the latest movement, another official said 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.

On Jan. 6, the North conducted it fourth nuclear test, despite warnings from the international community.

It detonated its first device in 2006, followed by two others in 2009 and 2013.

South Korean policymakers, meanwhile, said they believe the North could detonate another nuclear device whenever North Korean leader Kim Jong-un gives the order to do so. (Yonhap)