South Korean police said Wednesday they were investigating reports from several major broadcasters and banks that their computer networks had come under a concerted cyber attack. \"Members of our cyber team are investigating this now,\" a National Police Agency spokesman told AFP. Yonhap news agency said at least three broadcasters -- KBS, MBC and YTN -- and the Shinhan and Nonghyu banks reported that their computer networks had been paralysed, with the finger of suspicion likely to fall on North Korea. The North was believed to be behind two major cyber attacks in 2009 and 2011 that targeted South Korean government agencies and financial institutions, causing their networks to crash. The blame tables were turned last Friday when North Korea said it had been the victim of a \"persistent and intensive\" hacking assault by South Korea and the United States that took a number of its official websites offline for nearly two days. According to intelligence officials cited by South Korean media, North Korea is believed to have a cyber warfare unit staffed by around 3,000 people handpicked for their computer literacy. The South\'s military has a special alert level system called Infocon that reflects the current likelihood of an imminent cyber attack. With military tensions on the Korean peninsula at their highest level for years following the North\'s nuclear test last month, the Infocon level was recently raised from five to four -- with one being the top level. The Korea Internet Security Agency, a state watchdog, said it had recorded 40,000 cases of cyber attacks from foreign and domestic sources in 2012, up sharply from 24,000 in 2008. \"South Korea is an IT superpower with good infrastructure but remains relatively vulnerable to hacking,\" Park Soon-Tai, manager of the agency\'s hacking response team, told AFP in a recent interview.