SEOUL - The South Korean military sounded a possible high-level cyber attack alert Wednesday after computer networks crashed at major 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".

"An investigation is under way. Right now we cannot say whether they were attacked by hackers," an agency spokesman told AFP, adding that Internet service provider LG Uplus had also reported a network crash.

While there was no immediate confirmation of who or what was behind the multiple shutdown that occurred around 2:00 pm (0500 GMT), 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.

'We do not rule out North Korea being involved'

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.

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 intelligence officials cited by South Korean media, North Korea is believed to have a cyber warfare unit staffed by around 3,000 people handpicked for their computer literacy.

The Korea Internet Security Agency said it had 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.