(AP) -- Hackers attacked about 40 South Korean government and private websites Friday, prompting officials to warn of a substantial threat to the country's computers.
The South's National Cyber Security Center said they had seen signs of a "denial of service" attack, in which large numbers of computers try to connect to a site at the same time in an attempt to overwhelm the server.
A top South Korean cybersecurity company, AhnLab, said in a statement that the targets included websites at South Korea's presidential office, the Foreign Ministry, the National Intelligence Service, U.S. Forces Korea and some major financial institutions.
The Korea Communication Commission said websites had reported no immediate damage.
AhnLab spokesman Park Kun-woo said the attacks were similar to ones that have targeted South Korean websites in the past, in that they were denial of service attacks and largely targeted the same sites.
AhnLab said a computer user discovered a bug in their system Thursday night. After analyzing it, AhnLab found malicious software designed to attack websites and told the targets in advance so that they could prepare. As a result, Park said, there had only been a brief slowing of some of the websites.
AhnLab was providing free programs to repair infected computers.
Government officials have said that previous denial of service attacks on South Korean government websites were traced to China. It was not immediately clear where Friday's attack originated.
Park said people often point to China as the source of such attacks because a large amount of malware originates there. Malware is malicious software designed to access a computer without the owner's consent.
Cyberattacks on South Korea in 2009 were initially blamed on North Korea, but experts later said they had no conclusive evidence that Pyongyang was responsible.
South Korean media have previously reported that North Korea runs an Internet warfare unit aimed at hacking into U.S. and South Korean military networks to gather information and disrupt service.
Explore further: Second apparent leak of hacked celebrity nude pictures: US media