The defense ministry on Monday dismissed some civic groups' claims of finding an infiltration tunnel dug by North Korea near the capital city of Seoul as "groundless," vowing stern measures against false allegations that cause security jitters.

Last week, members of several conservative civic groups held a press conference in the northern city of Namyangju, just north of Seoul, and claimed that they have found an underground tunnel in the city dug by the communist country in order to infiltrate the South. 

"They said the Namyangju tunnel turned out to be linked to another one they allegedly found in the city of Yangju last month ... But the inspection by the National Forensic Science proved their claims groundless," Seoul's defense ministry spokesman Kim Min-seok said at a regular briefing.

Pointing out that the civic groups "committed irregularities by arbitrarily destroying the scene without reporting their findings to the authorities," he said the government will "take strong legal measures against any actions that stoke security concerns by making false claims."

The defense ministry has also said no pieces of evidence were found that show that the tunnel in Yangju was created by the communist North, noting that it is presumed to be a sinkhole or a tunnel created in the course of farmland reclamation.  

Heeding possibilities of the existence of such infiltration tunnels by Pyongyang, the South Korean military has launched operations to find them in dozens of suspicious spots near the demilitarized zone. 

Between 1974 and 1990, South Korea discovered four secret tunnels from North Korea believed to be aimed at launching a surprise attack on Seoul, but none have been found since 1990. (Yonhap)