A kidnapped Turkish truck driver was executed Monday by his captors, while Muslim clerics and Christian leaders condemned the deadly church bombings in their shared homeland the night before.

Religious leaders vowed to stand together to fight the violence that has raked Iraq since the U.S. invasion 15 months ago. But the execution of the hostage, along with new clashes in the Shiite holy city of Najaf between U.S. Marines and the militia of Moqtada Sadr, a firebrand cleric, highlighted the difficulties in restoring stability to Iraq.

Witnesses in Najaf said the Marines battled members of Sadr's Mahdi Army after attempting to approach the house of Sadr, whose followers waged a violent uprising against the U.S.-led occupation in April and May until a fragile truce was negotiated.

Marines surrounded Sadr's house with 10 Humvees and three tanks at 5:30 p.m., witnesses said. A gun battle broke out, they said, in which one woman was killed, and six militiamen and seven civilians, including children, were wounded.

Meanwhile, a video posted on the Internet showed the killing of Murat Yuce, the first Turk among the more than 70 foreigners abducted since spring to be executed. In the video, three men dressed in black, with their faces covered by white-and-red checkered scarves, stood behind a hostage, whose eyes were covered with a dark blue cloth. The man in the middle of the trio of captors twice shouted, "Allahu akbar!" or "God is great," then shot the hostage once in the head with a pistol. The man crumpled to the floor. Then the shooter repeated, "Allahu akbar!" and shot the hostage two more times in the head.

The captors, who call themselves the Monotheism and Jihad Group, said on Monday that they would release a Somali truck driver whose Kuwaiti employers agreed to stop operating in Iraq, according to al-Jazeera, the Arab satellite television network. The group has asserted responsibility for the beheadings of American businessman Nicholas Berg, South Korean translator Kim Sun Il and Bulgarian truck driver Georgi Lazov.

Edited footage of Yuce's killing was broadcast repeatedly on Turkish news channels on Monday. Within moments of the news of his death, the association representing Turkey's trucking industry announced it was halting deliveries to the U.S. military and other American clients in Iraq. Cahit Soysal, who heads the International Transportation Association, said the suspension would affect 200 to 300 trucks daily.

Turkish trucking is crucial to Iraq's fragile economy. More than 2,000 commercial vehicles a day enter the country from Turkey's Habur gate on Iraq's northern border, the vast majority of them importing goods that represented $800 million in trade during the first six months of this year, according to Ercument Aksoy, chairman of the Turkish-Iraqi Business Council.

Only convoys delivering goods to U.S. bases -- or gasoline and liquid propane to the Iraqi Oil Ministry -- are given military escorts from the border. The escorts take the form of two U.S. Humvees, one at the front of the convoy and another bringing up the rear, and are provided only to inbound shipments. Aksoy, who heads a company doing business in Iraq, said Yuce apparently was captured while returning to Turkey after leaving a U.S. base.

The only Muslim member of NATO, Turkey allows U.S. military flights through bases in its southeast and last year offered to supply troops to the U.S.-led force in Iraq. But only 9 percent of the Turkish population favors Turkey cooperating with the United States in Iraq, according to a July public opinion survey by Middle Eastern Technical University in Ankara, the capital. In fact, the poll found that 42 percent of those responding to the poll supported attacks on U.S. forces in Iraq.

"Nobody can predict what is going to happen in Iraq," Turkish Foreign Minister Abdullah Gul said Monday after Yuce's execution. "I hope that we can rescue the remaining Turkish hostages."

In Iraq, religious and government leaders condemned the bombings of five crowded churches on Sunday night, the first significant attacks against the country's Christian minority in recent memory. The bomb blasts outside four churches in Baghdad and one in the northern city of Mosul killed at least 11 people and wounded 47, according to the U.S. military.

Grand Ayatollah Ali Sistani, Iraq's most influential Shiite Muslim cleric, called the attacks "terrible crimes." In a statement, Sistani said the bombings "target the unity, stability and independence of Iraq."

"We think it's very important for our people to unite and cooperate among themselves, government and people, to stop the attacks on Iraqis and defeat the attackers," he said. "We stress the necessity of respecting the rights of Christian citizens as well as other minorities, including their right to live in peace in their country."

Iraq's interim government pledged money to help repair the churches, said Georges Sada, spokesman for the interim prime minister, Ayad Allawi. Sada said that Deputy Prime Minister Barham Salih had met with leaders of the Christian denominations on Monday and "expressed his deep condemnation" of the "terrorist incidents."

Iraq's interim president, Ghazi Yawar, also protested attacks against "any holy site, of any religion." In a statement, he said such acts showed "disrespect to the religious doctrine, and it is against all religions' traditions, especially Islam."

On a trash-strewn street off a main commercial thoroughfare in downtown Baghdad, Muslim neighbors of a Catholic convent tried to make sense of the blasts on Sunday night.

The women of the St. Anne Church for Nuns and Orphans have lived on the unmarked street since 1927, quietly worshiping in their small chapel and caring for orphans and elderly priests.

A sign near their sky-blue tin gate warns that visitors are not accepted after 8 p.m. The gate opens into a lush, green garden with a towering cross. Patio umbrellas advertising Pilsner beer provide shade for carefully tended plants.

Ekhlish Mansoor, 35, a nun, said the blasts did not deter her from going shopping Sunday night. "I don't feel afraid, because I have faith in God," said Mansoor, who ran away to a convent in her home town of Mosul when she was 10. "Life cannot be stopped."

Pauline Jimaa, 72, the mother superior at the convent, said she had been praying that God would bless the insurgents who attacked the churches. She said she wanted the bombers "to repent what they did" and God to "help them know their religion."

"It seems they don't follow their religion, because any religion doesn't accept what happened yesterday," Jimaa said.

Jimaa said six or seven families from the neighborhood came to offer support after the bombings. "They told me they will protect us, and they will be human shields to our building," she said.

Vick reported from Istanbul. Special correspondents Saad Sarhan in Najaf and Omar Fekeiki in Baghdad contributed to this report.

Nadwa Aziz, whose husband was killed in the bombing Sunday of a church in Mosul, is comforted by her sister-in-law, Mahasin Aziz, at a burial service.The family of Murat Yuce, a Turkish truck driver executed by his captors in Iraq, grieves at home in Ankara. Turkey's trucking association announced it would halt deliveries to American clients in Iraq in response to the killing.