At least three broadcasters -- KBS, MBC and YTN -- and two banks -- Shinhan Bank and Nonghyup -- reported to the National Police Agency (NPA) that their computer networks were entirely halted around 2 p.m. for unknown reasons, police said.
"Reports have been made simultaneously so we have dispatched investigators to the scene," a police officer handling cyber-terrorism at the NPA said, adding that officers will review all possibilities, including a cyber attack.
The national security office at Cheong Wa Dae has been briefed by police and other related agencies on the damage and possible causes of the network paralysis at TV stations KBS, MBC and YTN as well as Shinhan and Nonghyup banks, a key presidential official said.
"We are now trying to determine the cause of the network paralysis," the official said, adding that it has not yet been determined whether North Korea was involved in the incident.
As Seoul's telecom operators said that hacking was suspected in the unusual simultaneous network crashes at several broadcasters and banks, South Korea's military said it has upgraded its information surveillance status by one notch. The defense ministry said its computer network was operating normally as of late Wednesday afternoon.
The National Computing and Information Agency (NCIA), which oversees all the computer networks of government organizations, said its system was working normally without any errors.
"There was no abnormality detected at the computer networks of each administrative agency and local government," an official at the NCIA said, adding that all the intranet and Internet were operating normally.
South Korea has accused North Korea of carrying out a series of cyber attacks on the web sites of government agencies and financial institutions over the past few years, though the North denied the allegations.
A major daily newspaper was attacked by North Korean hackers in June 2012, while Nonghyup's computer networks crashed in September 2010 apparently under attack from the North, according to prosecutors and police.
Tensions on the Korean Peninsula have been running high in recent weeks as North Korea made repeated war threats in anger over ongoing joint military exercises between South Korea and the United States as well as a new U.N. Security Council resolution for its third nuclear test.
The networks at the broadcasters and banks were still out as of 4:15 p.m. Wednesday.
"The computer network has been paralyzed since 2 p.m. and we cannot do any business," an official of public broadcaster KBS said.
Another official of cable broadcaster YTN said not only the computer network but also editing equipment for the broadcasting system has also been paralyzed.
"We are figuring out the exact cause and it is expected to affect the broadcast," the official added.
Shinhan Bank said it has suffered from disruptions with its computer network since around 2:20 p.m., which slowed down business at each branch. The bank also said that other services such as Internet banking, mobile banking via smart phones, and automated teller machines have also been affected.
Nonghyup Bank said its telecommunications network suffered a disruption at several branches but other services were operating normally.