"The enemy has evolved — a little bit more lethal, a little more complex, a little more sophisticated, and in some cases, a little bit more tenacious," said Lt. Gen. Ricardo Sanchez. "The evolution is about what we expected to see over time."
American forces are being attacked 15-20 times a day, counting roadside bombs, mostly in Baghdad and the surrounding Sunni strongholds to the west and north of the capital, Sanchez said.
Since May 1, when the U.S. declared the end of major combat, an estimated 90 soldiers have died in combat, according to an Associated Press tally. A total of 314 American service members have died since the war started March 20, according to the U.S. Defense Department.
Sanchez blamed the changing nature of the conflict on an influx of militants and other terrorist elements coming in from Syria and northern Iran to join the core resistance of Saddam loyalists.
"We believe there is, in fact, a foreign fighter element. There is a terrorist element focused on the coalition and international community in general and the Iraqi people to try to disrupt the progress being made," he said.
In other developments:
In the latest violence, U.S. soldiers came under fire Thursday near the Fallujah mayor's office and killed one of their attackers, an American officer said, while a witness said a U.S. convoy was attacked southeast of the volatile city.
Those incidents came a day after three American soldiers were killed in separate attacks as the U.S.-led coalition faced an increasingly sophisticated resistance movement.
None of the Americans was hurt in the attack by three gunmen in Fallujah, a major city 30 miles west of Baghdad in the so-called "Sunni Triangle," but two girls were injured in the crossfire, Lt. Col. Brian Drinkwine said.
A check by The Associated Press at the town's two hospitals showed one dead and four wounded — a policeman, a 17-year-old boy who underwent surgery for an abdominal wound, and a mother and her 4-year-old daughter. All were in stable condition.
Drinkwine said the attack was aimed at the city building.
"While we were conducting a meeting in the city council building (mayor's office), we were fired upon. We returned fire and killed one enemy," Drinkwine said.
Shortly before the attack, a fuel tanker in a U.S. convoy near Amiriyah, southeast of Fallujah, was hit by a mine or roadside bomb, according to Mohammed Hamid, who lives nearby. He said a soldier in the passenger seat of the cab pulling the tanker was killed and the driver was wounded. The military had no information on that attack.
Twenty miles to the east in Khaldiyah, a roadside bomb exploded as a U.S. convoy was passing, but did not damage the American vehicles.
Witness accounts of the Fallujah attack were at odds with those of the military, with some claiming the gunmen fired from a passing car on a U.S. foot patrol. Others said a single gunman attacked from the street.
Ali Jassim, commander of the Fallujah Protection Force, also said the dead man was not an attacker but an innocent bystander. He said policeman Mohammed Muafaq, 27, was shot in the hip.
Walid al-Jumaly, a tire shop owner, said more than 10 soldiers were walking across the main street in front of the mayor's office and an adjacent U.S. Army post when a man stepped from a side street, shouted "God is great!" and started firing with an assault rifle.
He said the Americans used tear gas and returned fire.
Afterward, residents of the Euphrates River city said they were happy the soldiers came under attack, calling the assailant a freedom fighter.
Assou Nadim Hamid, a policeman himself and brother of one of eight Fallujah police mistakenly killed by U.S. troops Sept. 12, voiced anger at the Americans.
"Whenever they come inside Fallujah, they will be attacked. Saddam Hussein is gone. But now we have the same kind of regime," he said.
A bomb was found at the mayor's office last week and defused. U.S. troops routinely are in the office to coordinate reconstruction projects in the region.
Fallujah, a wedge of land west and north of Baghdad, has been the scene of repeated attacks by resistance fighters opposed to the American occupation.
In Tikrit, the military said the Baath Party official was arrested overnight near Baqouba. His name was not released, but the military said he was believed to have been helping Izzat Ibrahim al-Douri, a longtime Saddam confidant and one of the most senior members of the former regime still at large.