Seoul's Unification Ministry has expressed regret over a North Korean defector group's attempt to send anti-Pyongyang leaflets across the border, and reaffirmed its stern response. 

It added that an investigation is currently ongoing into the group, called Fighters for a Free North Korea.

The ministry said related agencies including the police will work together to take stern measures against such acts.

It reiterated the stance that sending leaflets and other articles across the border raises inter-Korean tension and threatens the lives and safety of border residents.

Park Sang-hak, the chief of Fighters for a Free North Korea, claimed that six of its members released large balloons carrying leaflets, pamphlets, one dollar U.S. bills and SD cards from Paju, Gyeonggi Province Monday night.