WASHINGTON — The powerful Internet attack that overwhelmed computers at U.S. and South Korean government agencies for days was even broader than initially realized, pestering the White House, the Pentagon and the New York Stock Exchange and shutting down other official Web sites.

Targets of the most widespread cyber offensive of recent years also included the National Security Agency, Homeland Security Department and State Department, the Nasdaq stock market and The Washington Post, according to an early analysis of the malicious software used in the attacks.

The cyber assault on the White House site had "absolutely no effect on the White House's day-to-day operations," said spokesman Nick Shapiro.

Preventative measures kept whitehouse.gov "stable and available to the general public," Shapiro said, but Internet visitors from Asia may have experienced problems.

South Korean intelligence officials believe the attacks were carried out by North Korea or pro-Pyongyang forces, but many experts in cyberwarfare said it was simply too early to know where the offensive originated.

Many of the U.S. government targets appear to have successfully blunted the sustained computer assaults. But others, such as the Treasury Department, were knocked off-line at times.

Two government officials acknowledged that Treasury's site was brought down, and said the agency had been working with its Internet service provider to resolve the problem. The officials spoke on condition of anonymity because they were not authorized to speak on the matter.

As of last night, Shapiro said, "all federal Web sites were back up and running."

Ed Donovan, a spokesman for the U.S. Secret Service, said that the cyber attacks slowed access to the agency's Web site, which operates on the same computer server as Treasury's. But Secret Service's site remained in operation despite the crippling effects of the cyber offensive, Donovan said.

"Our site was never knocked down, but it was slowed down at points," Donovan said. He added that Secret Service's "operational side" was not affected by the attacks.

State Department spokesman Ian C. Kelly told reporters that the department's state.gov Web site has been under attack since July 5.

"It's still ongoing but I'm told it's much reduced now," Kelly said.

The Associated Press obtained the target list from security experts analyzing the attacks. It was not immediately clear who might be responsible or what their motives were.