The fighters appear united in their goal - to kill Americans. But Pentagon officials say none of the attacks have been launched simultaneously or in a coordinated manner at multiple targets. Such coordination, the officials said, might indicate planning beyond individual groups of Iraqis working independently of one another.

"Most of the attacks have been targets of opportunity," one senior Pentagon official said. "We see nothing of a higher hand trying to organize these attacks into something you would actually call a guerrilla campaign, and certainly not a military campaign."

What one military officer called "the leaderless resistance" in Iraq is believed to be made up of former members of Saddam Hussein's Republican Guard and secret service, as well as Baath Party members, fedayeen guerrillas and foreign fighters who came to join the battle against the U.S.-led invasion.

"These people have no future in a post-Saddam Iraq," another Pentagon official said. "They have nothing else to do, and nothing else to lose. They are forming in small groups. They are united in killing Americans. But there is no central coordination that we've seen."

In addition to increasing U.S. military casualties, the goals of a guerrilla war would be to exhaust the American public's patience with its postwar role in rebuilding Iraq and to intimidate Iraqis cooperating with the U.S. reconstruction effort by showing that Saddam's loyalists can still strike, another Pentagon official said.