Major Korean websites including those of Cheong Wa Dae, Chosun.com and Naver suffered simultaneous hacker attacks on Tuesday evening, causing the Internet to become disconnected for about four hours. Websites of leading U.S. agencies such as the White House could not be accessed from Korea either.

The attack by an unidentified hacker of unknown nationality inconvenienced more than 10 million subscribers.

In the past, the Internet Data Center, which controls servers of various websites, was hacked and some services were suspended, but this was the first time such a large-scale attack hit websites of leading agencies simultaneously.

After 6 p.m. on Tuesday, major Korean websites including Chosun.com, Cheong Wa Dae, the National Assembly, the Defense Ministry, Naver's e-mail and blog services, Auction, and Shinhan and Korea Exchange banks were impossible to connect to or became extremely slow.

There were also temporary server connection problems to U.S. government websites such as the White House and the State Department. Those connections were given priority and went back to normal at 10:30 p.m., but some websites continued to see connection problems far into the night.

The National Intelligence Service and the Internet industry insiders believe a hacker group launched what is known as a "distributed denial of service attack" (DDoS) on leading websites in several sectors.

An NIS official said the agency is trying to ferret out the hackers and work out a response.