At least 18 Shiite pilgrims are reported to have been killed by a car bomb in Baghdad.

45 people are also said to have been left wounded when a vehicle, parked in a Say-di-yah neighbourhood, exploded after midday today.

The Sunni extremist Islamic State group have since claimed responsibility for the bombing. It said the assault was carried out by a suicide bomber, but Iraqi officials have denied that. Authorities say that the pilgrims had gathered to commemorate the anniversary of the death of a revered imam, when the fatal incident occurred.

The attack comes one day after two car bombs exploded in the southern Iraqi city of Samawah, killing at least 31 people and wounding 52. Islamic State have claimed to be behind this bombing as well.

Thousands of Shiite followers have been travelling towards the northern Baghdad neighborhood of Kadhimiyah this week, where the 8th century Imam Moussa al-Kadhim is buried. Security forces have blocked major roads in Baghdad in anticipation of attacks against pilgrims who traditionally travel on foot from different parts of Iraq.

According to the United Nations, at least 741 Iraqis were killed in April due to persistent violence. In its monthly report issued on Sunday, the U.N. mission to Iraq claimed that a total of 1,374 Iraqis were wounded that month.

The capital, Baghdad, remains the worst-hit area, after almost 250 civilians were killed and 642 were left wounded in April.