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South Korean defence ministry's cyber wing had come under attack in what is suspected to have been launched by North Korean hackers. Although the cyberattack took place in September, Seoul have confirmed its intranet was attacked on Tuesday (6 December).

The extent of the infiltration is yet to be determined as South Korean authorities are ascertaining the magnitude of the data leak. If confirmed, this would mark the first-ever breach of its defence ministry's cyber command.

"The military formed a cyber investigative team to look into this matter and found that some military data – including confidential information – has been leaked. It appears to be a North Korean act," the defence ministry said.

The affected network has been isolated to prevent the spread of the malware. The cyber wing of the defence ministry was set up in January 2010 as part of South Korea's counter-hacking attempts specifically mounted by Pyongyang.

"It seems the intranet server of the cyber command has been contaminated with malware. We found that some military documents, including confidential information, have been hacked," a source from the defence ministry told Yonhap news agency. The ministry had earlier tried to play down the attack and said that chances of a hack are quite low.

North Korea is believed to have groomed a dedicated army of hackers to launch several cyberattacks in the past. Seoul has blamed the North for previous such hacking campaigns which brought down the websites of state-run institutions. The latest development has come at a time when tensions are high in the Korean peninsula.