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U.S. government Web sites — including those of the White House and the State Department — have been under attack since the Fourth of July, along with financial and commercial sites like Yahoo Finance and the New York Stock Exchange, cybersecurity experts said Wednesday.

The Department of Homeland Security, which is one of the targets, according to a security expert, confirmed that the attacks were taking place.

Web sites in South Korea, including the president’s, were targets of the same attack, said Jose Nazario, manager of security research at Arbor Networks in Ann Arbor, Michigan.

The Washington Post said it, too, had been attacked.

There is some indication that the attack comes from China, Nazario said, but he added that he could not be certain of the origin. Even if it is coming from China, it would be difficult to determine whether officials or individual hackers were responsible.

He said the attack was of moderate size, involving “a few tens of thousands” of infected computers “around the world.”

“We measured a peak of 25 megabits/second” in data transmission, he said, calling it “about the size of a big PowerPoint presentation, well in the garden variety of what we see.”

But other cybersecurity experts said that even if the current attack was not particularly worrisome, it was a window into potentially more serious problems.

Jim Lewis of the Center for Strategic and International Studies in Washington said that the attack was simple and primitive but that the fact that it worked on some agencies shows the government is still “disorganized.”

Some parts of the government were able to “beat this off,” while others haven’t, he said.