NEW : Attack traced to IP addresses in China , officials say . North Korea has staged similar attacks in the past , expert says . Military , broadcasters safe ; government , unaffected , Yonhap reports . South Korean military steps up its cyberdefense efforts in China .
NEW : Attack traced to IP address in China , officials say . North Korea has staged similar attacks in the past , expert says . Banks , broadcasters targeted ; government networks unaffected , Yonhap reports . South Korean military steps up its cyberdefense efforts in response .