Alarming messages published on known Al Qaeda (search) Web sites are claiming the terror network is about to launch its biggest strikes ever - and are warning Muslims to leave the cities of New York, Washington and Los Angeles.

The statements monitored by U.S. government agencies, as well as by a Washington-based terrorist research group, are part of the alarming increase in "chatter" (search) among terrorist operatives in recent weeks that played a role in the Bush administration's decision to raise the nation's threat alert to high (orange).

Counterterrorism expert Rita Katz, director of the SITE Institute, said her organization has recently found and translated statements on Al Qaeda Web site Al-Lewa - Arabic for "The Banner" - that are promising new attacks on U.S. soil in the coming weeks.

Katz said a posting two weeks ago quoted an Al Qaeda spokesman identified as Abu Issam al-Yamani as saying, "The next Al Qaeda attacks will be most violent and will target the U.S." and urged Muslims "to leave the country if they don't wish to die as a result of a Jihadist operation."

A second message was posted on the same Web site last Thursday, from a group calling itself the Islamic Bayan Movement.

"Our Muslim brothers in America, this is our final warning. We ask you, as fast as you can, to leave the following cities immediately: Washington D.C., New York City, Los Angeles," the message said.

Katz said she noticed Al Qaeda's stepped-up cyber propaganda began Nov. 15 after the terror bombings in Istanbul, Turkey when a known Al Qaeda group warned in a communiqué that the "death cars will not stop."

Katz said the electronic vitriol has continued almost daily, and just last night a message was published in which Al Qaeda's mouthpiece, the Global Islamic Media Society, took delight that Americans are now "living in a state of anxiety and constant fear."

It ended with, "You will forget about Manhattan events. Be prepared."

U.S. government officials say it is impossible to determine whether the message writers may be involved in plotting attacks

But these officials told The Post last night that the CIA, FBI and National Security Agency consider the messages "pieces of the overall puzzle."