BAGHDAD -- An explosives-laden car detonated Monday in the Iraqi capital, killing at least 13 Shiite pilgrims commemorating the death anniversary of a revered Imam, officials said.

There was no immediate claim for responsibility for the attack, but it bore the hallmarks of the Islamic State extremist group which sees Shiite Muslims as apostates. The militants frequently target commercial areas and public spaces in mainly Shiite towns and neighbourhoods.

The parked car bomb exploded shortly after midday in Baghdad's southwestern Saydiyah neighbourhood, a police officer said, adding that at least 28 others were wounded.

A medical official confirmed the causality figures. Both officials spoke on condition of anonymity as they were not authorized to release information.

Tens of thousands of Shiite faithful have been making their way this week to the northern Baghdad neighbourhood of Kadhimiyah, 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.

While IS has suffered a number of territorial defeats in the past year, the group still controls significant pockets of territory in Iraq's north and west, including the country's second largest city of Mosul, estimated to still be home to more than 1 million civilians. Militants have recently increased attacks inside Baghdad in what officials say is an attempt to distract from their recent battlefield defeats.

According to the United Nations, at least 741 Iraqis were killed in April due to ongoing violence, a sharp decline from the previous month. In its monthly report issued on Sunday, the U.N. 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, followed by the northern province of Ninevah, which is almost entirely controlled by the Islamic State group, with 72 killed and 30 wounded.