Sheik Ghalib Ali al-Zuhairi was a member of the Association of Muslim Scholars, an influential Sunni clerics group that has called for a boycott of nationwide elections scheduled for Jan. 30.
He was shot as he was leaving a mosque in the town of Muqdadiyah and died in the local hospital, said police Col. Raisan Hussein. Muqdadiyah is about 60 miles north of Baghdad.
A day earlier, unknown gunmen assassinated another prominent Sunni cleric in the northern city of Mosul — Sheik Faidh Mohamed Amin al-Faidhi, who was the brother of the group's spokesman. It was unclear whether the two attacks were related.
The U.S. military had no immediate comment on whether there were any American casualties in the attack in Samarra, north of Baghdad. Hospital officials there said the bomb exploded off a road connecting Samarra and nearby Duluiya and U.S. troops returned fire, killing one man.
In the same town, mortar rounds slammed into a residential neighborhood near a U.S. military base on Tuesday, wounding two youngsters, said Police Maj. Saadoun Ahmed Matroud.
In other developments:
The violence in Iraq has spread despite this month's Fallujah offensive, which was aimed at bringing stability ahead of January's national elections.
On Monday the U.S. Embassy said a bomb was discovered on a commercial flight inside Iraq. No further details were released and the statement did not say whether the affected flight had arrived or was preparing to depart. Aircraft flying into and out of Baghdad have been fired on frequently by insurgents.
Militant Sunni clerics have called on Iraqis to boycott the Jan. 30 elections, angered by the Fallujah action and last week's U.S.-Iraqi raid on a Baghdad mosque, which left three dead and about 40 others arrested.
Still, Iraq's interim prime minister expressed confidence Monday that most Iraqis would participate in the election.
"The forces of darkness and terrorism will not benefit from this democratic experience and will fight it," Ayad Allawi told The Associated Press. "But we are determined that this experiment succeeds."
Allawi, a secular Shiite hand-picked by the Americans last June, said he believed that only "a very small minority" would abstain during the election "for one reason or another."
He is expected to run for a seat in the assembly, which would then choose the government.
The United States is anxious that the election go ahead as planned, hoping that an elected government widely accepted by the Iraqi people will take the steam out of the insurgency still raging in Sunni areas of central, western and northern Iraq as well as the capital.
As the election approaches, U.S. commanders in Iraq probably will expand their troops by several thousand. Army units slated to depart are also being held back until after the election. There are now about 138,000 U.S. troops in Iraq.
U.S. officials are concerned that a boycott could deprive the new government of legitimacy in the eyes of the Sunni Arabs, who make up an estimated 20 percent of the nearly 26 million population. The majority of Shiites, believed to form 60 percent of the population, strongly support elections.
In Egypt, where 20 nations have gathered for an international conference on Iraq, members have committed themselves to supporting the U.S.-backed Iraqi interim government and its war against insurgents.
The gathering, which included many who had opposed the U.S.-led invasion of Iraq, represented hard-won acknowledgment of the need for international cooperation to deal with the consequences of the war.
"It is a world duty to save Iraq from its tragic situation," said Egyptian Foreign Minister Ahmed Aboul Gheit
The Sharm el-Sheikh delegates did not set a deadline for withdrawing U.S.-led forces from Iraq — despite a push by France and some Arab countries. However, a draft communique obtained by The Associated Press, does say the mandate of foreign forces is "not open-ended."
Meanwhile, the first independent aid group to enter the Iraqi city of Fallujah after two weeks of fighting had to turn back before delivering any assistance because of security concerns, the Red Cross said Tuesday.
Ahmed Rawi, a spokesman for the International Committee of the Red Cross, said he was still awaiting a full report from Iraqi Red Crescent on what it encountered, but was able to confirm that the convoy had entered Fallujah on Monday.
He said the convoy, which consisted of ambulances and three trucks loaded with blankets, drinking water and first-aid kits, saw a few civilians before having to turn back.
"They did not deliver any assistance," Rawi said, adding that he was waiting for the convoy's report. "We want to see what they saw inside. We want to see their evaluation of the conditions."