A car bombing in Baghdad has killed at least 18 Shiite pilgrims commemorating the anniversary of the death of a revered Shiite imam, Iraqi officials say.

A police officer said the parked explosives-laden car detonated shortly after midday in Baghdad's southwestern Saydiyah neighbourhood.

He added that 28 other people were wounded in the explosion.

Isis have claimed responsibility for the attack online. The militant group sees Shiite Muslims as apostates.

Shiite pilgrims make their way to Kadhimiya to mark the death of Imam Moussa al-Kadhim before the car bomb attack in Baghdad, Iraq, 2 May, 2016 (Reuters)

Militants have recently increased attacks inside Baghdad, in what officials regard as an attempt to distract from their recent battlefield defeats.

Tens of thousands of Shiite faithful have been making their way to the northern Baghdad neighborhood of Kadhimiyah this week, where the 8th century Imam Moussa al-Kadhim is buried.

Security forces blocked major roads in Baghdad in anticipation of attacks.