SEOUL A hacking attack that brought down three South Korean broadcasters and two major banks has been identified by most commentators as Pyongyang flexing its muscles as military tensions on the divided peninsula sky-rocket.
Officials in Seoul traced Wednesday's breach to a server in China, a country that has been used by North Korean hackers in the past. That reinforces the vulnerability of South Korea, the world's most wired economy, to unconventional warfare.
One government official in Seoul directly blamed Pyongyang, although police and the country's computer crime agency said it would take months to firmly establish responsibility.
Jang Se-yul, a former North Korean soldier who went to a military college in Pyongyang to groom hackers and who defected to the South in 2008, estimates the North has some 3,000 troops including 600 professional hackers in its cyber unit.
Jang's alma mater, the Mirim University, is now called the University of Automation. It was set up in the late 1980s to help North Korea's military automation and has a special class for professional hacking.
The North's professional "cyber-warriors" enjoy perks such as luxury apartments for their role in what Pyongyang has defined as a new front in its "war" against the South, Jang told Reuters.
"I don't think they will stop at a temporary malfunction. North Korea can easily bring down another country in a cyber warfare attack," Jang said.
Like much about North Korea, its true cyber capabilities are hard to determine. The vast majority of North Koreans have no access to the Internet or own a computer, a policy the regime of Kim Jong-un strictly enforces to limit outside influence.
The nominee to be the next South Korean intelligence chief told MPs recently the North was suspected of being behind most of the 70,000 cyber attacks on the country's public institutions over the past five years, local TV channel YTN reported.
North Korea recently threatened the United States with a nuclear attack and said it would bomb South Korea in response to what it says are "hostile" war games in the South by Washington and Seoul.
Threats to bomb the mainland United States are empty rhetoric as Pyongyang does not have the capacity to do so and its outdated armed forces would lose any all-out war with South Korea and Washington, military experts say.
That makes hacking an attractive, and cheaper, option.
"North Korea can't invest in fighter jets or warships, but they have put all their resources into raising hackers. Qualified talent matters to cyber warfare, not technology," said Lee Dong-hoon, an information security expert at Korea University in Seoul.
Much of North Korea's limited funds also go into its nuclear and ballistic missile programmes.
Wednesday's attack hit the network servers of television broadcasters YTN, MBC and KBS as well as two major commercial banks, Shinhan Bank and NongHyup Bank. South Korea's military raised its alert levels in response.
About 32,000 computers at the organisations were affected, according to the South's state-run Korea Internet Security Agency, adding it would take up to five days to fully restore their functions.
It took the banks hours to restore banking services. Damage to the servers of the TV networks was believed to be more severe, although broadcasts were not affected.
South Korea's military, its core power infrastructure and ports and airports were unaffected.
Investigations of past hacking incidents on South Korean organisations have been traced to Pyongyang.
"There can be many inferences based on the fact that the IP address is based in China," said the South Korean communication commission's head of network policy, Park Jae-moon. "We've left open all possibilities and are trying to identify the hackers."
North Korea has in the past targeted South Korea's conservative newspapers, banks and government institutions.
The biggest hacking effort attributed to Pyongyang was a 10-day denial of service attack in 2011 that antivirus firm McAfee, part of Intel Corp, dubbed "Ten Days of Rain". It said that attack was a bid to probe the South's computer defences in the event of a real conflict.
North Korea last week said it had been a victim of cyber attacks, blaming the United States and threatened retaliation.
"North Korea is able to carry out much bigger attacks than this incident such as stopping broadcasts or erasing all financial data that could panic South Korea," Lee of Korea University said. (Additional reporting by Jack Kim, Narae Kim, Hyunjoo Jin, Joyce Lee and Se Young Lee; Editing by David Chance and Dean Yates)