Car bombs in two cities and fighting in a third killed dozens of Iraqis Saturday, and U.S. military officials said they suspected that Iraqis loyal to former president Saddam Hussein had formed a marriage of convenience with an alleged al Qaeda operative to carry out widespread attacks.

In Hilla, a Shiite Muslim city 55 miles south of Baghdad, a car bomb exploded on a busy commercial street, killing at least 31 people, according to a hospital official and Iraqi police. In the northern city of Irbil, another car bomb wounded a local official, killed a shopkeeper and hurt 10 bystanders, according to local reports.

[The U.S. military said on Sunday it believed 40 people were killed in Hilla in two car bomb blasts, not one as originally thought, the Reuters news agency reported.]

"Why are they doing this here? This is terrorism, and there is no reason for those people to do this on this street," said Jabar Ali, a bystander in Hilla. He said he saw a man park his car in front of a tea shop and walk away. When the shop owner yelled at him to move the car, the man shouted back, "Shut up," and the car exploded.

The blast destroyed surrounding stores and set other cars afire. At the main hospital in Hilla, Nazar Abdul Mehdi said the dead included women and children and that 62 people had been wounded.

There was no immediate assertion of responsibility for the blast, and witnesses said there were no political targets in the area, suggesting that the bomb may have been intended to create panic among civilians.

The explosion added to a long list of attacks by insurgents ahead of Wednesday's scheduled handover of limited authority to an Iraqi interim government. A key figure in those attacks, U.S. officials say, is Abu Musab Zarqawi, a Jordanian who has asserted responsibility for much of the violence, including a fusillade of attacks Thursday that killed more than 100 Iraqis and three U.S. troops.

U.S. officials said Saturday that they believed Zarqawi, whom they have described as a confederate of al Qaeda, was cooperating with fugitive armed supporters of the former Hussein government and would escalate car bombings and other attacks before Wednesday.

Though both groups are composed mostly of Sunni Muslims, Iraqis loyal to Hussein and his secular Baath Party tend to be political opposites of Zarqawi and al Qaeda, who advocate a worldwide Islamic holy war. But U.S. officials said they believe the groups apparently have begun to cooperate in an effort to derail the process of formally ending the U.S. occupation.

"It may be we are seeing these two groups converging," said a military official. "Zarqawi's network is growing. He is a guy who wants to go out and grab new business."

Zarqawi's supporters continued to grab the spotlight Saturday, threatening to behead three Turkish workers taken as hostages.

Army Brig. Gen. Mark Kimmitt, the chief U.S. military spokesman in Iraq, appealed to Iraqis to bear in mind the $10 million reward offered for information on Zarqawi's whereabouts. U.S. forces bombed three houses in Fallujah last week believed to be used by Zarqawi, but Kimmitt said he had no reason to believe Zarqawi was a casualty in any of the three attacks, which killed at least 62 people.

"He remains the number one target inside this country," Kimmitt said.

A video statement claiming to be from Zarqawi's group and sent to the al-Jazeera satellite television network showed the three Turks, who the group said would be executed in 72 hours unless Turkey pulled its nationals out of Iraq. Turkey has no troops in the country, but Turkish contractors and employees are working on reconstruction projects.

There was no immediate comment from the Turkish government, which is hosting a summit meeting of NATO leaders in Istanbul beginning Monday. President Bush arrived in Ankara, Turkey's capital, Saturday.

Zarqawi and his followers asserted responsibility for the beheadings of a South Korean contractor last week and an American businessman last month.

In Baqubah, 35 miles northeast of Baghdad, six guerrillas were killed in clashes with American troops, U.S. officials said. They announced no U.S. casualties from those battles, but said an American soldier had died early Saturday morning after being shot in an ambush in Baghdad.

Special correspondent Saad Sarhan in Hilla contributed to this report.