The New York Stock Exchange, Nasdaq and US Bank Web sites are among the victims of a sustained computer attack targeted at American and South Korean organisations.
According to reports, South Korean intelligence officials believe North Korean or pro-Pyongyang forces are behind the distributed denial of service (DDoS) attacks that have hit dozens of Web sites over the last few days.
The perpetrators are understood to have used a virus to create a botnet made up of tens of thousands of infected computers, which then overloaded targeted Web sites with millions of requests.
US government sites, including those of the Federal Trade Commission and the transportation department were felled by the attack over the weekend. In South Korea, the president's site and those of the ministry of defence and national assembly were hit.
Financial institutions have also been targeted. According to a list of affected sites published by a Korean blogger, Nyse, Nasdaq, US Bank, Korea Exchange Bank and Shinhan Bank were all among the victims.
The Homeland Security department's US computer emergency readiness team has issued a notice to federal departments and others about the problems and given advice on how to deal with them.