The South Korean military raised its cyber attack warning level Wednesday 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 2:00 pm (0500 GMT), but the main finger of suspicion is likely to point at Pyongyang.
Wednesday's 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 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.
The National Computing and Information Agency, which oversees all the computer networks of government organisations, said its system was working normally.
The presidential Blue House said the national security advisor had been briefed by police on Wednesday's network paralysis and his office was investigating the possible causes.
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.
Seoul's Korea Internet Security Agency 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.
China, North Korea's main patron which has angrily denied being behind a spate of cyber attacks on US interests, said the incident in South Korea showed the importance of a collective response to IT threats.
"China would like to work with other countries based on mutual respect and mutual trust in constructive cooperation in this field," foreign ministry spokesman Hong Lei said.
And in a phone call to South Korean President Park Geun-Hye, China's new leader Xi Jinping said Beijing was willing to help promote "reconciliation" between Seoul and Pyongyang amid the current tensions.