BAGHDAD – Insurgents targeted passenger buses north of Baghdad on Tuesday, as a suicide bomber killed at least eight people west of Mosul and gunmen seized 21 men traveling through Diyala province.

The latest bloodshed highlighted the slow-going, punch-counterpunch U.S.-led campaign against al-Qaida in Iraq, more than a month after Iraq’s prime minister said he expected the fight for Mosul would be a “decisive battle.”

The Americans view the northern campaign as a chance to subdue al-Qaida in Iraq in areas surrounding Mosul, a major transportation hub which the military has described as the terror group’s last urban stronghold.

Tuesday’s bombing, 40 miles west of Mosul, struck a bus heading from that city to the Syrian capital of Damascus.

Violence has rattled much of the northern region in recent days.

The U.S. military said it killed seven al-Qaida in Iraq members and captured three insurgents during a firefight Monday east of the town of Khan Bani Saad, near the Diyala provincial capital of Baqouba. Three soldiers were wounded, it said.

Elsewhere in Diyala, police said gunmen in civilian clothes stopped two buses at a fake checkpoint on a highway in the Adeim area, 45 miles north of Baqouba, then took the buses and kidnapped 21 men. They later released three women, said an officer, who spoke on condition of anonymity because he wasn’t authorized to release the information.

Iraqi army and U.S. military versions of the bus bombing varied somewhat, with the Iraqis saying that nine people were killed and eight wounded, and the U.S. authorities placing the death toll at eight, with eight wounded.

In Karbala, 50 miles south of Baghdad, the annual spectacle of Arbaeen was on display Tuesday – under heavy security.

The commemoration marks the end of the mourning period following the anniversary of the death of Imam Hussein, who was slain in a seventh century battle and is buried in Karbala. Iraqi and U.S. authorities have said at least 8 million pilgrims will join the ceremonies by the time they reach their peak Wednesday and Thursday. They will be joined by 40,000 troops, snipers and plainclothes security officers.

“I came 10 days ahead of time to serve the pilgrims of Imam Hussein, defying all circumstances and terrorist acts,” said 62-year-old pilgrim Mohammed Hussein.

Associated Press writers Sinan Salaheddin and Hamid Ahmed contributed to this report.