BAGHDAD, Iraq -Iraqi officials announced that the FBI would help an Iraqi team investigate Monday's killing of the head of the U.S.-appointed Iraqi Governing Council.

The killing was a major setback to American efforts to stabilize Iraq just six weeks before the June 30 handover of sovereignty.

In Baghdad, Interior Minister Samir Shaker Mahmoud al-Sumeidi said an Iraqi team would investigate the killing of Saleem with FBI assistance.

Brig. Gen. Mark Kimmitt, the chief military spokesman in Iraq,

had said the bombing had the "classic hallmarks" of Abu Musab al-Zarqawi, a Jordanian-born militant with links to al-Qaida.

But on Tuesday, he said another group may be to blame "because of methodology in some of the techniques that were used." He did not elaborate.

A previously unknown group, the Arab Resistance Movement, claimed responsibility in a Web site posting.

Al-Zarqawi is believed responsible for many of the vehicle bombs in Iraq in recent months and for the beheading last week of Ui.S. civilian Nicholas Berg.

Meanwhile, Iraq's most respected Shiite cleric urged both U.S. soldiers and a radical cleric's militia Tuesday to withdraw from two Shiite holy cities where fighting has raged near some of Shia Islam's holiest shrines.

A statement released in Grand Ayatollah Ali al-Husseini al-Sistani's name urged Iraqis not to travel to Najaf to join protests called by cleric Muqtada al-Sadr. Instead, he said, Shiites should join rallies elsewhere to demand that Najaf and Karbala "be rid of all armed manifestations."

However, the statement, which al-Sistani's aides distributed to reporters after nighttime skirmishes in Najaf, did not include the ayatollah's personal seal nor was it posted on his Web site, as is customary with religious decrees, or fatwas, which are binding on his followers.

An aide to al-Sistani, speaking on condition of anonymity, said the ayatollah wants both the Americans and al-Sadr's al-Mahdi Army out of the holy cities in southern Iraq but has avoided an explicit call because he knows neither side is prepared to accept it.

Brig. Gen. Mark Hertling, deputy commander of the U.S. Army's 1st Armored Division, said there were indications that al-Sadr was trying to bring in fighters to reinforce his militia in Karbala, about 50 miles south of Baghdad.

"We are doing our best to intercept those people," Hertling said at Camp Lima, a military base on the outskirts of the city.

Kimmitt

said coalition forces had agreed to reduce their presence in the Baghdad neighborhood of Sadr City, an al-Sadr stronghold, in exchange for a cessation of attacks on coalition troops. The deal was made Sunday in a meeting with local leaders, and the district has calmed, though there were two mortar attacks, he said.

The U.S.-led coalition is struggling to contain an insurgency in Sunni areas north and west of Baghdad, as well as an uprising in a Shiite district of Baghdad and the Shiite heartland to the south led by al-Sadr.

In Lebanon, the leader of the Hezbollah guerrilla group accused U.S. soldiers on Tuesday of desecrating holy shrines in Najaf and Karbala, and called on Muslims to fight to the death to defend the sanctities.

"By attacking holy sites, they are attacking all Muslims and all Shiites worldwide," Sheik Hassan Nasrallah told thousands of supporters in a Beirut southern suburb.

At least five Iraqi insurgents were killed during clashes in Karbala Tuesday, according to Capt. Noel Gorospe, a U.S. military spokesman there.