Hackers have attacked around 40 South Korean government and private websites.
Targets include websites at the presidential office, the foreign ministry, the national intelligence services and some major financial institutions, according to US reports.
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Government officials have warned of a substantial threat to the country's computers, with the National Cyber Security Centre reporting signs of a denial-of-service attack.
During a computer system check on Thursday, South Korean IT security firm AhnLab found malicious software which was designed to attack websites.
The firm warned targets in advance of the attacks and reports say there has been only a brief slowing of some of the websites without any damage caused.
The US reports say South Korean media have previously reported that North Korea runs an internet warfare unit aimed at hacking into US and South Korean military networks to gather information and disrupt service.
Cyberattacks in 2009, which crashed systems and infected computers with malware in the US and South Korea, were blamed on North Korea at the time, although the allegations have never been proven.