The struggle against ISIL has rekindled communal ties between sectarian communities in Iraq.
A car bombing targeting Shia pilgrims commemorating the death anniversary of a revered imam in Baghdad has killed at least 18 people, according to Iraqi officials.
A parked explosives-laden car blew up shortly after midday on Monday in the southwestern Sayadiyah neighbourhood.
The Islamic State of Iraq and the Levant group (ISIL, also known as ISIS) claimed responsibility for the attack.
Tens of thousands of Shia Muslims have been making their way this week to the northern Baghdad neighbourhood of Kadhimiyah, where the 8th-century Imam Musa al-Kadhim is buried.
Kadhim, the seventh of 12 imams revered in Shia Islam, died in AD799.
The pilgrimage has in recent years turned into a huge event that brings Baghdad to a standstill for days.
Security forces had previously said they would be blocking major roads in Baghdad on Monday and Tuesday to prevent potential attacks on pilgrims.
Series of attacks
On Saturday, a car bomb killed at least 23 people and injured 38 others near the Iraqi capital.
The bombing, also claimed by ISIL, targeted an open-air market frequented by Shia in Nahrawan.
According to the UN, at least 741 Iraqis were killed in April owing to ongoing violence, a sharp decline from the previous month.
In its monthly report issued on Sunday, the UN mission to Iraq put the number of civilians killed at 410, while the rest were members of the security forces. A total of 1,374 Iraqis were wounded that month, it added.
In March, at least 1,119 people were killed and 1,561 wounded.
The capital, Baghdad, remains the worst-hit area, with 232 civilians killed and 642 wounded in April.