Attacks on federal computer networks are common, ranging from nuisance hacking to more serious assaults, sometimes blamed on China. U.S. security officials also worry about cyber attacks from al-Qaida or other terrorists.

The widespread attack was "loud and clumsy," which suggests it was carried out by an unsophisticated organization, said Amit Yoran, chief executive at NetWitness Corp. and the former U.S. government cybersecurity chief. "This is not the elegance we would expect from sophisticated adversaries."

Web sites of major South Korean government agencies, including the presidential Blue House and the Defense Ministry, and some banking sites were paralyzed Tuesday. An initial investigation found that many personal computers were infected with a virus ordering them to visit major official Web sites in South Korea and the U.S. at the same time, Korea Information Security Agency official Shin Hwa-su said.

Associated Press writers Hyung-Jin Kim in Seoul, South Korea; Andrew Vanacore in New York; Pamela Hess in Washington and Pan Pylas in London contributed to this report.