Federal authorities and security companies are warning American businesses to protect themselves next week during a planned "Labor Day Strike" from Chinese hackers upset over the recent spy plane incident.

The National Infrastructure Protection Center, the FBI's cybercrime arm, said Thursday that "Chinese hackers have publicly discussed increasing their activity" from April 30 to May 7. Those days coincide with two major Chinese holidays, International Workers Day and Youth Day.

May 7 also is the two-year anniversary of the bombing of China's embassy in Belgrade, Yugoslavia.

American hackers have vandalized scores of Chinese Web sites, frequently adding obscene or racist anti-China screeds. Next week's attack would be in retaliation, according to U.S. security firms.

A smattering of American sites have already been attacked by hackers claiming to be from China. They have left messages such as "hack the USA" and "For our pilot Wang!!! For our China!," according to security company Vigilinx. Wang Wei is the name of the Chinese pilot who died after crashing into an American spy plane.

Antivirus company Symantec Corp. also is warning its clients about attacks from China, and several network administrators have reported on security e-mail lists about increased prodding of their networks originating from China.

There is no evidence that the attacks are sanctioned or organized by the Chinese government.

About 65 Chinese Web sites have been vandalized by American hackers since the U.S. reconnaissance plane crash-landed in China, according to Web site Attrition.org, which monitors Web site defacements.

While Web defacements are usually little more than harassment -- the only U.S. government site believed to be attacked recently by a Chinese hacker was an obscure Navy site -- Vigilinx officials warn that more serious attacks could occur.

"It has the potential to escalate into something very damaging if emotions run unchecked," said Vigilinx chief executive Bruce Murphy.