Cyberattacks aimed at jamming South Korean Web sites continued Thursday with more private sites being targeted. Thursday's attacks hit one government and six commercial sites, although only two were affected for more than a few hours. At least 27 government and commercial sites in South Korea and the United States have been hit since the denial-of-service attacks began July 4.

"I'm growing increasingly concerned about the cyber world and the attacks, whether they are from individual attackers or from state entities," Adm. Mike Mullen, chairman of the U.S. Joint Chiefs, said at the National Press Club July 8, although he declined to address specifics of the recent attacks.

"We need to raise more people who are capable in this area," Mullen added. "The fiscal 2010 budget was a very comprehensive approach to the future, including a significant investment … in the cyber world."

Among sites hit in South Korea are the Kookmin and Shinhan banks, Web portals Naver, Daum, and Paran, and the Chosun Ilbo newspaper, as well as government sites including the president's Blue House, the Defense Ministry and the National Assembly.

The Korea Communications Commission has undertaken what it calls "an aggressive distribution of vaccine programs against the attack," but the government has faced public criticism for not reacting faster. Large financial losses are expected as a result of aborted online purchases and financial transactions.

Suspicion in South Korean defense and intelligence circles is falling on North Korea as the source of the attack. It "appears to be thoroughly planned and executed by a specific organization or on a state level," the National Intelligence Service said in a statement, although it did not name any particular state. In May Korean news agency Yonhap reported that North Korea was putting more money, staff and equipment into a cyber warfare unit to hack into U.S. and South Korean military networks.

Some American experts believe it's more likely that the attacks came from a non-government group sympathetic to North Korea. They also consider that the attack was not sophisticated at all but a brute-force attack known as "distributed denial of service." Thousands of computers running Microsoft Windows are infected with a virus that allows the programmer to link them in a virtual or robot network, known in the trade as a "botnet." The botnet is then directed to deluge target sites with requests for information that overwhelm their capacity and cause them to crash.

Government and corporate sites in Georgia were hit with denial-of-service attacks last year right before the Russian invasion. Dozens of sites in Estonia were hit with massive attacks in April and May of 2007 that closed many for almost three weeks, in the wake of violent clashes between Estonians and ethnic Russians over the removal of a Soviet war memorial from the capital, Tallin.

U.S. officials said the recent attacks involved 50,000 to 65,000 infected computers. A botnet could hijack a million computers. While the attack shut down sites at the Treasury Department, the Federal Trade Commission, the Transportation Department and the Secret Service, other sites with larger server capacities, such as the White House, Pentagon and New York Stock Exchange, weathered the assault with little difficulty.

The denial-of-service attack has proved a disruptive nuisance and highlighted discrepancies between government agencies in their level of preparedness to meet them. But they are not the main reason that Secretary of Defense Robert Gates announced on June 23 the creation of a Cyber Command within the Department of Defense, nor what has led him to budget for the training of 200-plus new cyber security officers annually.

The big worry is the ability of cyber spies to steal sensitive information or take control of critical systems. In April it was reported that computers based in China had hacked into the network of developers working on the F-35 Joint Strike Fighter project and copied several terabytes of data on its electronics and design. The same month software was discovered in the electrical grid left there by cyber spies based in Russia, China and other countries who had penetrated the system. If undetected the software could have been used to damage or disrupt the grid's operation.