The South Korean military raised its cyber attack warning level after computer networks crashed at major TV broadcasters and banks, with initial suspicions focused on North Korea.
The Korea Internet Security Agency, a state watchdog, 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 cyber wing 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, which occurred around 2pm (1600 AEDT), but the main finger of suspicion is likely to point 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.
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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 South Korean Defence Ministry said it had raised its five-level "Infocon" cyber threat 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 level.
"We do not rule out the possibility of North Korea being involved, but it's premature to say so. It will take time to figure out," Defence Ministry spokesman Kim Min-Seok said.
Shinhan Bank said in a statement that it had been forced to turn away branch customers, while its internet banking and ATM operations were also badly affected. Its network was partially restored after two hours, the bank said.
A KBS labour union spokesman said all the broadcaster's computers had crashed simultaneously.
"We're on air, but journalists are having difficulties filing stories as they cannot access the network," he said.
According to South Korean intelligence officials, North Korea is believed to have a cyber warfare unit staffed by around 3,000 people handpicked for their computing prowess.
