WASHINGTON A hacker war between the United States and China heated up as the White House website was hit with a E-mail "bombs," and dozens of U.S. and Chinese sites were defaced, security experts said.

The escalation came as Chinese hackers Monday began a week-long campaign of attacks targeting U.S. government and commercial Internet sites, in retaliation for what they see as assaults by pro-U.S. hackers, AFP quoted experts as saying.

The campaign, dubbed "The Sixth Network War of National Defense," officially began at 1200 GMT as China began its May Day celebrations, according to experts who monitor hacker activity.

According to transcripts of a Chinese hacker group meeting obtained by AFP, the cyberattacks will peak later this week.

Some of the sites targeted in addition to the White House include those operated by the FBI, NASA, Congress as well as media outlets such as the **** New York Times ****, the **** Los Angeles Times ****, CNN and MSNBC.

"This is unique because of the level of organization," said Jerry Freese, director of Intelligence for the New Jersey technology security firm Vigilinx.

"We would never go as far to say it is state-sponsored, but it's certainly state-tolerated," said Freese.

Michael Cheek of the Virginia security firm Idefense said the White House website was being hit with a flood of E-mails in a tactic called "E-mail bomb attacks" in an effort to crash its mail server.

"They've developed an E-mail flood tool," he said, adding that the attacks appeared to originate from Beijing.

The Chinese hackers "have claimed success" in bringing down the mail server, Cheek said, "but I have no indication that's actually the case." There was no immediate comment from the White House.

Cheek said Chinese hackers Monday had hit 18 U.S. websites, while pro-U.S. hackers had hit 23 sites in China.

Over the month of April, Cheek said pro-U.S. hackers had defaced at least 350 Chinese websites, to 37 for pro-Chinese hackers targeting American sites.

"This is retaliation against the Americans for their perception that American hackers have been attacking them," Cheek said.

Hackers worldwide have apparently been choosing up sides in the cyber-battle.

Pro-U.S. hackers are now being supported by hackers in Saudi Arabia, Pakistan, India, Brazil, Argentina, and Malaysia. pro-China hackers are receiving support from hackers in Korea, Indonesia and Japan, said Marquis Grove, of Security News Portal, a hacker news site.

Grove feared that the alliances signal a possible worldwide hacker war. "This will escalate the cyberwar to a level of intensity never seen before," Grove said.

Grove added that another type of attack a so-called "Ping" attack, was planned for Friday against the main White House website.

The FBI had warned system operators of the pending attacks last week. But FBI spokesman Debbie Weierman said Monday she could not comment on any reports of the latest cyberattacks.

Freese said the U.S.-China tensions rose in recent weeks following the U.S. spy plane incident, which he said prompted a series of U.S. cyberattacks on Chinese sites.

A U.S. EP-3 surveillance craft collided with a Chinese F-8 fighter over the South China Sea on April 1, resulting in the loss of the Chinese aircraft and pilot. The 24 U.S. crewmembers held on the China's Hainan island were released after almost two weeks.