BAGHDAD — A car bomb in a restive town north of Baghdad on Friday killed six Iranian pilgrims, officials said, the latest in a spike in violence that has sparked fears of all-out sectarian war.

Anotherr 23 worshippers were wounded in the attack, which struck their bus as it was passing through the town of Muqdadiyah en route from the Iranian border to the holy Shiite city of Najaf.

Najaf, which lies south of Baghdad, is home to a shrine to a revered figure in Shiite Islam.

Shiite Muslims visiting holy shrines and religious sites form the backbone of Iraq's tourism industry, with the vast majority of such visitors coming from Iran.

When completing a tour of Iraq's key Shiite religious sites, pilgrims typically visit Najaf, nearby Karbala, Baghdad, and Samarra, the latter of which lies north of the capital.

But Sunni militants, including those linked to al-Qaida, view Shiite Muslims as apostates and often target them for attacks. No group immediately claimed responsibility for Friday's bombing, however.

Attacks in Iraq have risen sharply, with May the country's deadliest month since 2008, as persistent political disputes have given fuel and room for militants to increase their activities.

There has been a heightened level of violence since the beginning of the year, coinciding with rising discontent in the Sunni Arab minority that erupted into protests in late December.

The U.N. envoy to Iraq has warned that the violence is "ready to explode."