Hackers have stolen and leaked personal details of 40,000 US soldiers and two million members of South Korea's ruling political party, according to a report by news agency Reuters.
The news follows a series of cyber attacks on both South Korean and North Korean targets yesterday, the anniversary of the start of the Korean in 1950. Affected organisations included the South Korean president's office and North Korea's official news agency.
Hacktivist group Anonymous claimed responsibilty for the attacks on North Korean targets, but denies targetting any South Korean sites.
Some sources are assuming that the North Korea, which has launched cyber attacks against the South in the past, lies behind these apparent counter attacks. US-based website NKnews, however, has suggested that Anonymous may lie behind the attacks on the South in order to stage "a modern-day reconstruction of events by first hacking South Korean websites".
The South Korean government has said that no government, military or banking systems were compromised in the latest wave of attacks. It is not clear where the leaked data may have come from.
In March, a more serious, malware-borne attack took government and banking systems offline for days. The South Korean government blamed the North's spy agency for the attack.
South Korea is arguably the world's most advanced digital culture, with 93% of households having access to the web.
However, the recent spate of cyber attacks has evidently eroded confidence in the digital economy: a global survey published this week by Big Brother Watch found that 71% of South Koreans believe that "consumers are being harmed by big companies gathering large amounts of personal data for internal use", a much higher proportion than any other country in the world.