Nearly simultaneous suicide bombings at three military checkpoints in northern Iraq killed as many as 18 people Tuesday, police said, and dozens were hurt in Baghdad by a car bomb apparently aimed at a police convoy. Meanwhile, U.S. and Iraqi troops launched an offensive against insurgents in the volatile northern town of Tall Afar.

The triple bombings outside the town of Hawija, about 130 miles north of Baghdad, occurred within a few minutes of one another at checkpoints manned by Iraqi soldiers, said Gen. Ahmed Obeidi of the Iraqi police.

At 9:30 a.m., a suicide bomber tried to drive his car onto a U.S. military base in Baggara, three miles west of Hawija, but could not get past an Iraqi army checkpoint, Obeidi said. The bomber detonated his car at the checkpoint and killed one Iraqi soldier.

Minutes later, another suicide car bomber struck a checkpoint in Aziziya, on the north side of Hawija, killing two Iraqi soldiers.

The deadliest bombing occurred soon afterward in Dibis, a town two miles west of Hawija. Iraqi soldiers grew suspicious of a car in a line of vehicles at the checkpoint there, Obeidi said. When the soldiers approached the car, he said, it exploded, killing 11 Iraqis, including five soldiers, and setting more than 10 cars ablaze.

After the attacks, shops in Hawija closed and people, including police officers and soldiers, poured into the main hospital to donate blood, which had been in short supply, said Jasim Hamad, a physician at the hospital. Hamad said three children younger than 5 were among those killed.

The Associated Press reported late Tuesday that the death toll had reached 18.

The U.S. military, meanwhile, reported that two Marines had been killed Monday in separate roadside bombings outside the western city of Fallujah. And a Sunni Muslim cleric, Salam Kardici, was found shot to death Tuesday in the southern port of Basra, where a Shiite Muslim cleric was assassinated last week. Kardici had been abducted Sunday.

Attacks on Iraqi and foreign security forces have accounted for a large proportion of the more than 850 killings in Iraq since a transitional government led by Prime Minister Ibrahim Jafari took office in late April.

The car bombing Tuesday morning in Baghdad wounded 28 people, including a policeman, according to a hospital official who spoke on condition of anonymity.

Witnesses in the neighborhood of Shula, in northern Baghdad, said the bomb apparently was intended to hit a convoy of police officers who passed the same spot every morning on their way to set up a security checkpoint nearby.

"We knew it would happen here one day," said Esmail Thamir, 42, who owns a real estate office 15 yards from where the car containing the bomb was parked. "They shouldn't set up their checkpoints in these places. They know they are targets. They shouldn't expose civilians to their danger."

The joint U.S.-Iraqi military assault in Tall Afar involved hundreds of troops, as well as armored vehicles and attack helicopters, the Associated Press reported. Further details were not immediately available late Tuesday, but a U.S. military spokesman in Baghdad confirmed that an operation was underway in Tall Afar.

Over the weekend, local officials and religious and tribal leaders from Tall Afar held talks with Iraqi and U.S. military representatives that yielded a four-point agreement aimed at curbing insurgent and sectarian violence. The accord acknowledged, however, that failure to achieve peace would make a large-scale military incursion unavoidable.

In Baghdad, a government spokesman backed away from recent statements that ousted president Saddam Hussein would face trial within two months on charges focusing on 12 crimes during his tenure. Laith Kubba, spokesman for the prime minister, said any information about the prosecution would henceforth come from the special tribunal formed to try Hussein.

Last week, President Jalal Talabani had said Hussein would be tried within two months, a time frame that Kubba appeared to confirm on Sunday, when he also said the trial would focus on 12 counts among more than 500 on which Hussein could be charged.

But on Tuesday, Kubba said the tribunal "assured me that they have a press officer and that they would like the information to be taken from them." A tribunal spokesman could not be reached by telephone, but the Reuters news agency quoted a statement by the panel asserting that no trial date had been set.

Ani reported from Hawija. Special correspondent Omar Fekeiki in Baghdad contributed to this report.

Iraqi soldiers inspect one of the car bombing sites near Hawija, about 130 miles north of Baghdad. All three explosions occurred at Iraqi checkpoints.