Major banks and TV broadcasters suffered massive computer network failures on Wednesday afternoon. It is unclear who was behind the attack.

The computer networks of broadcasters KBS, MBC and YTN simultaneously failed at around 2:15 p.m. Computers suddenly shut down or froze and could not be rebooted until late afternoon.

Shinhan Bank's computer network suffered the same fate. Computers at headquarters and all branches failed or had files deleted automatically, while operations ranging from banking and ATMs to online transactions ground to a halt. Shinhan managed to restore the network about two hours later.

Jeju Bank, Nonghyup, NH Life Insurance and NH Fire also suffered massive network failures until late afternoon. No damage was reported in government or military computer networks.

The Korea Communications Commission said a malicious code was spread through servers in companies that automatically update PC vaccine programs. Cyber security experts said hackers opted for what is known as an "Advanced Persistent Threat" to knock down the networks -- a more sophisticated way of bringing down entire computer systems than the so-called distributed denial-of-service attacks South Korea suffered previously, which simply overload the systems.

The government and military here are focusing on the fact that the cyber attacks came just five days after North Korea threatened revenge for two days of hacking attacks apparently suffered by the state-run Rodong Sinmun daily and Korean Central Television.

North Korea also threatened to attack major South Korean media last year. "This could be the start of a full-fledged cyber war," said one intelligence official. "Future attacks could target the government's computer network, nuclear power plants, railways and other national infrastructure."

Cyber security experts called for a thorough check to assess the vulnerability of computer networks of the state-run organizations and other major institutions.

President Park Geun-hye was briefed on the situation on Wednesday afternoon by Kim Jang-soo, the nominee for presidential security adviser, and told officials to find out what caused the failures.

The Defense Ministry said it raised the cyber threat level, or INFOCON, by one notch.

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