The threat has seen Defense Minister Kim Kwan-jin, who is known for his hardline stance against Pyongyang, given a military security escort when he leaves his office.
"The defense minister is a key official and we're always keeping close tabs on his security," one source said. "When he has to leave the office for functions and other commitments, we do take extra care."
Military and intelligence officials were moving to establish whether any would-be assassins were North Korean agents or other foreign nationals sent by Pyongyang, another source said.
Kim, a former army general, assumed his post last December and has been strident in his views on responding forcibly to perceived provocation from the North.
Pyongyang's state media has responded by dubbing him an "anti-North Korea confrontation maniac."