Shia pilgrims pray in the holy city of Karbala, Iraq. A bomb targeting pilgrims Wednesday killed at least 20 and injured more than 100. (Hadi Mizban/Associated Press)

A bomb on a parked motorcycle exploded early Wednesday on the outskirts of the Iraqi holy city of Karbala, killing at least 20 Shia pilgrims and wounding over 100 others, officials said.

The blast was the latest in a string of attacks this week that have targeted pilgrims making their way to an important Shia religious observance in Karbala, raising fears of a spike in attacks when the pilgrimage culminates Friday.

The bomb went off at about 11 a.m. local time in an area known as Ibrahimia, near the east entrance — one of three — into Karbala, 80 kilometres southwest of Baghdad.

At least 108 people were wounded in the attack, an official said, adding that the area was clogged with pilgrims and the number of casualties could increase. A hospital official in Karbala, also speaking on condition of anonymity, confirmed the number of dead and wounded.

Earlier Wednesday, two roadside bombs targeting Shia pilgrims exploded in Baghdad, killing one and wounding seven others, a security official in the capital said. The official said the first attack took place at about 6:15 a.m. in western Baghdad, killing one person and wounding three. A second explosion in southwest Baghdad wounded four.

The official spoke on condition of anonymity because he was not authorized to release the information.

Wednesday's attacks follow a suicide bombing two days earlier north of Baghdad that killed 54 Shia pilgrims.