South Korea's military has raised its alert level in response to what officials say is a widespread cyberattack that crippled the networks of several major broadcasters and top banks.
Seoul's Defense Ministry says it cannot rule out the involvement of North Korea, which has a history of carrying out cyber warfare against the South. But South Korea's Internet and Security Agency says there is so far no evidence of an attack coming from outside the country.
The shutdowns began Wednesday afternoon South Korean time, paralyzing the computers of at least three broadcasters - KBS, MBC and YTN. Computers at two banks - the Shinhan Bank and the Nonghyup Bank, were also hit, as were some of the country's ATMs. The country's stock markets have not been affected, though South Korean stocks fell in trading following the news.
South Korea's presidential office spokesperson Kim Haeng said the government has started a "thorough" investigation and is preparing countermeasures.
"Currently, we have formed a cyber crisis countermeasure headquarters with a combined task force team including civilians, government and military and dealing with it in real time."
As of late Wednesday, some of the computers were back online. But many others employees were not yet able to access the computers at their workstations. Officials say it could take several days to identify who was responsible for the attack, which comes at a particularly tense time on the Korean peninsula.
North Korea has used increasingly violent rhetoric against the U.S.-backed South in recent weeks, threatening to wage all-out war after the United Nations passed sanctions in response to its latest nuclear test.
Pyongyang is believed to have an elite cyber warfare unit that was reportedly behind computer attacks on South Korean government agencies and financial institutions in 2009 and 2011. South Korea's military says it was not affected by the outage.
The attack comes just days after North Korea accused the United States and South Korea of launching a computer attack on several of its websites, which mysteriously suffered a prolonged outage last week.