At least 23 Iraqis have been killed and dozens wounded in a string of attacks as Iraqi Shi'ites observe Ashura - the holiest day of their religious calendar.

The deadliest attack happened on a Baghdad bus, when a suicide bomber boarded and detonated his explosives. Authorities say at least 17 people were killed and more than 40 injured.

In another attack, a suicide bomber rode a bicycle into a group of mourners at a Baghdad funeral, setting off a blast that killed four and wounded 38.

The U.S. military says an American soldier was also killed in one of the Baghdad bombings, but did not specify which.

And a suicide attack in Baquba killed two people outside an Iraqi National Guard base.

The attacks have raised fears of a repeat of the violence seen during Ashura last year, when more than 170 people were killed during religious commemorations in Baghdad and the Shi'ite holy city of Karbala.