Insurgents fired a rocket-propelled grenade at a U.S. military vehicle in the restive town of Fallujah (search), wounding an NBC News employee, and U.S. forces arrested a former Iraqi colonel as part of a sweep against remnants of Saddam Hussein's regime.

In southern Iraq, American soldiers detained the U.S.-appointed mayor of the Shiite-dominated city of Najaf, accusing him of kidnapping and corruption, and arrested 62 of his aides. The mayor, Abu Haydar Abdul Mun'im (search), had been been deeply unpopular among Najaf residents because of his background in Saddam's military.

U.S. and British troops have been trying to win favor among the Shiite Muslims (search) in the south, which until recently had seen little of the anti-coalition violence that has plagued American forces in mostly Sunni central Iraq.

In the western city of Hadithah, 150 miles northwest of Baghdad, a huge explosion at an ammunitions depot killed at least three people and injured four, according to initial reports from the U.S. military. It was not immediately clear what caused the blast.

Early Sunday, U.S. troops launched their latest major sweep in central Iraq — codenamed Operation Sidewinder (search) — aimed at rooting out Saddam loyalists and others thought behind the near daily attacks on Americans.

In one arrest, troops detained a colonel from Saddam's Baath Party along with five other people, the military said Monday, without providing details. The statement said at least 319 Iraqis have been detained in several operations, including Sidewinder, across Iraq since Sunday.

In Monday's attack in Fallujah, the grenade hit an Avenger air defense vehicle — a Humvee equipped with a missile launcher — injuring the NBC freelance employee traveling in the vehicle but leaving the soldiers unscathed, U.S. Central Command (search) said.

NBC News producer Carol Grisanti identified the injured employee as Australian Jeremy Little, a television sound man. He was evacuated to a combat support hospital and was in stable condition, the Centcom statement said.

Soon after the attack — as Little was being taken away — a white pickup truck plowed into an armored personnel carrier there to help evacuate any casualties, Centcom said. The three Iraqis in the truck were killed.

The crash appeared to be an accident, not an attack. At a Fallujah hospital, Zoheir Ali told The Associated Press that his son-in-law, Qahtan Hashem, was driving the truck, rushing two neighbors — a son and his sick father — to the hospital ahead of an 11 p.m. curfew. Ali said he believed Hashem did not intend to hit the vehicles.

The latest sweep — carried out by troops of the 4th Infantry Division and Task Force Ironhorse — covers an area of central Iraq stretching from the Iranian border to the areas north of Baghdad. The region has become "the nexus of paramilitary activity in central Iraq," the military said in a statement.

The operation is expected to last for several days, according to military officials in Camp Boom, near Baqouba, 35 miles northeast of Baghdad.

U.S. forces in central Iraq have been plagued by sneak attacks on their positions and patrols. At least 63 American soldiers have been killed in attacks and accidents in Iraq since major combat was declared over on May 1.

With Operation Sidewinder in its second day, there were no reports of U.S. casualties — nor was there any indication that the operation had netted any of Iraq's most wanted fugitives.

The American forces arrested a man in Khalis, 45 miles north of Baghdad, suspected of recruiting others to attack U.S. troops. In Dojima, a town where residents recently polished the still-standing portrait of Saddam, police raided the homes of alleged Saddam loyalists suspected of hiding caches of arms, including rocket-propelled grenades — the weapon of choice in many recent ambushes.

U.S. officials in Washington have said repeatedly that Iraqi resistance to American rule is not centrally organized. But commanders on the ground painted a different picture.

Young called the resistance northeast of Baghdad "an organized effort." And Capt. John Wrann said: "It's got to be a coordinated thing."

The arrest of the Najaf mayor appeared to be separate from Operation Sidewinder.

U.S. troops sealed off Abdul Mun'im's office in Najaf, arrested him and his aides and replaced him with Haydar Mahdi Mattar al-Mayali, a former deputy in the mayor's office.

Abdul Mun'im was installed by the Americans shortly after they entered Najaf, 110 miles southwest of Baghdad, in April. In addition to kidnapping, Mun'im stands accused of holding hostages, pressuring government employees to commit financial crimes, and attacking a bank official.

In recent weeks, residents of Najaf have held demonstrations against Mun'im, accusing him of links to Saddam's dissolved Baath Party.

Meanwhile, one of the most senior Shiite clerics in Iraq, Ayatollah Ali al-Sistani issued a fatwa, or religious ruling, denouncing U.S. administrators' plan to create a council to draw up a new constitution for Iraq. Al-Sistani said a council handpicked by the Americans was "fundamentally unacceptable."

In the fatwa, dated Saturday and posted on al-Sistani's Web site, the ayatollah called for elections to pick delegates to a constitutional convention and a referendum to approve any constitution it draws up.

Meanwhile, U.S. troops quietly returned to Syria five border guards who had been wounded in a June 18 attack by American forces at the Iraqi-Syrian border. The handover, announced Monday, came as both countries aimed to avoid further strains to their relations.

U.S. ground troops and warplanes attacked an Iraqi convoy on the border, believing it was carrying top Saddam loyalists. One Iraqi was killed and the wounded Syrian guards — caught up when the fighting spilled across the border — were taken into American custody.

On Monday, the London-based human rights group Amnesty International (search) said it had evidence that the U.S. military violated international law by subjecting prisoners to "cruel, inhuman or degrading" conditions at its Iraq detention centers.

The group said in a report that hundreds of Iraqis held at U.S.-run tent camps and former Iraqi government prisons have been denied the right to see families or lawyers or have a judge review their detention.

The report coincides with a United Nations conference on human rights that began Monday in Baghdad. The conference, which focuses on abuses during Saddam's rule, will coordinate investigations into the regime's alleged killings of some 300,000 Iraqis.

A U.S. military spokesman in Baghdad said military officials could not comment on the Amnesty report because they had not received it.