South Korea's military has detected no signs of underground tunnels allegedly dug by North Korea in two areas near Seoul, officials said Friday, vowing stern action against those who make false claims.

Several conservative civic groups have claimed that they have found 12 underground tunnels -- six in the northern city of Yangju and another six in nearby Namyangju -- dug by the North to infiltrate the South.

"In order to dissipate suspicions, the military scoured the areas for four days from Monday, mobilizing more than 100 military and civilian personnel as well as 26 units of equipment," the Joint Chiefs of Staff (JCS) said in a press release.

"We drilled a dozen holes in the suspicious regions and carried out diverse analyses, including sound waves and the composition of rocks. But not a single sign has been found to indicate that such underground tunnels exist," it said. 

Criticizing the civic groups for their refusal to accept the military's explanation, the JCS vowed to take strong legal action against false claims.

"Making groundless allegations repeatedly not only results in a waste of the military budget but causes security jitters among the people," the JCS said. "We will take strong legal action against the civic groups."

Later Friday, the JCS is scheduled to hold a media briefing about their findings at the sites.

Between 1974 and 1990, South Korea discovered four secret tunnels believed to have been dug by North Korea to launch surprise attacks on Seoul. No tunnels have been found since 1990. (Yonhap)