A car bomb exploded Wednesday in the northern Iraqi city of Mosul, killing four civilians as the local government acknowledged it has yet to persuade frightened Christians to return to the homes they fled, police said.

A police officer in Ninevah province said the bomb went off in a parked car in a predominantly Sunni area of this city, where violence has continued against Christians and other religious minorities despite months of U.S. and Iraqi military operations to chase out extremists.

Mosul officials said few of the nearly 10,000 Christians chased from their homes earlier this month are returning to the city, despite government pledges of financial support and protection.

Every Christian family that comes back to Mosul would receive 1 million Iraqi dinars — about US$865 — on orders of the prime minister, said Jawdat Ismaeel, a local migration official.

Lt. Gen. Riyadh Jalal Tawfiq, the Iraqi military commander for Ninevah province, said the government was fulfilling its responsibility to "give protection to every family that returns home." He said checkpoints and foot patrols were helping to improve the security situation in Mosul.

"We urge other families to come back," Tawfiq said. "We will ensure their protection."

Islamic extremists have frequently targeted Christians and other religious minorities since the 2003 U.S.-led invasion, forcing tens of thousands to flee Iraq. However, attacks had declined as areas became more secure after a U.S. troop buildup, a U.S.-funded Sunni revolt against Al Qaeda and a Shiite militia cease-fire.

Sunni insurgents are believed to be behind the recent campaign that has driven out roughly half of the Christian population in Mosul, the country's third-largest city, located 225 miles (360 kilometers) northwest of Baghdad. A Christian community has lived there since the early days of Christianity.

In Cairo, Arab League chief Amr Moussa condemned the attacks.

"We can't remain silent as brutal crimes are being committed against the Christian Iraqis," Moussa said in a statement.

In Baghdad, meanwhile, police and hospital officials said three separate blasts killed a sick man being transported in an ambulance and wounded 11 others. All officials spoke on condition of anonymity because they were not authorized to disclose the information.

The bombings highlight the continued security challenges in Iraq amid a national debate over the future role of American troops in the country.

Iraq's Cabinet on Tuesday decided to ask the United States for changes in a draft agreement that would keep the U.S. military here for three more years, as Shiite lawmakers warned the deal stood little chance of approval as it stands.

Without the deal, which must be ratified by Iraq's parliament, there would be no legal basis for the U.S.-led military mission after the current U.N. mandate expires on Dec. 31, forcing hard decisions in Baghdad and Washington on the future of the unpopular war.

On Wednesday, an influential Iraqi cleric living in Iran issued a fatwa — or religious decree — against the pact, calling it "haram," which in Arabic means "forbidden" by Islam.

Ayatollah Kazim al-Hosseini al-Haeri, believed to be a mentor of anti-U.S. Iraqi Shiite cleric Muqtada al-Sadr, called the proposed agreement a "sin God won't forgive."

The comments carry significance as Iraqi Prime Minister Nouri al-Maliki, a Shiite, is trying to build wide backing for the agreement. His party's main ally in Iraq's ruling Shiite alliance, the Supreme Islamic Iraqi Council, maintains close ties to Iran, which strongly opposes the deal.

The agreement calls for U.S. troops to leave Iraqi cities by the end of June and withdraw altogether from the country by Dec. 31, 2011, unless the government asks them to stay. It would also provide limited Iraqi jurisdiction over U.S. soldiers and contractors accused of major, premeditated crimes committed off post and off duty.