The South Korean military raised its cyberattack warning level yesterday after computer networks crashed at major TV broadcasters and banks, with initial suspicions focused on North Korea.

The state-run Korea Internet Security Agency said computer networks at three TV broadcasters - KBS, MBC and YTN - as well as the Shinhan and Nonghyup banks had been "partially or entirely crippled".

LG Uplus, an internet service provider, also reported a network crash. An investigator from the specialist cyberwing of the national police agency said the shutdown appeared to have been triggered by a "virus or malicious code", suggesting a concerted hacking operation.

There was no immediate confirmation of who or what was behind the multiple shutdown, but the main finger of suspicion is likely to be pointed at Pyongyang.

The crash came days after North Korea accused South Korea and the United States of being behind a "persistent and intensive" hacking assault that took a number of its official websites offline for nearly two days.

The South Korean Defence Ministry said it had raised its five-level "Infocon" cyberthreat alert status from four to three.

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 only recently raised from five to four - with one being the top threat level.

Shinhan Bank said it had to turn away branch customers, while its internet banking and ATM operations were also badly affected. The network was restored after four hours.