MOSUL, Iraq, (Reuters) – A suicide car bomber killed 21 people and wounded 72 as worshippers left a Shiite Muslim mosque in northern Iraq on Friday, police said, the same day a series of bombs in the capital killed six Shiite pilgrims.

A week ago a series of blasts outside Shiite mosques in Baghdad killed 31 people. Shiite religious gatherings are frequent targets of Sunni Islamist militants such as al Qaeda, who consider Shiites heretics.

The attack in northern Iraq took place just north of the city of Mosul, 390 km (240 miles) north of Baghdad.

The insurgency in Iraq has waned in the last 18 months, but has remained stubborn in Mosul and a few other areas. Insurgents have been able to hide out in the remote areas around Mosul, and have exploited divisions between Mosul’s Arabs and Kurds.

Bombings and shootings are reported almost daily in Mosul.

In Baghdad, roadside bombs exploded as minibuses carrying Shiite Muslims home from pilgrimage a day earlier passed by.

Two roadside bombs struck two minibuses in separate incidents in the poor Baghdad Shiite district of Sadr City.

Three people were killed and eight wounded in one blast, and one person was killed and seven were wounded in the other.

Another roadside bomb struck a minibus in east Baghdad, killing two people and wounding nine, a hospital source said.

Hundreds of thousands of pilgrims poured into Iraq’s holy city of Kerbala on Thursday to mark the birth of Imam Mohammed al-Mehdi, a Messiah-like figure Shiites believe vanished centuries ago and will return to bring peace on earth.

The event was the second big religious gathering in Iraq since the withdrawal of U.S. combat troops from urban centres on June 30, which thrust Iraqi security forces into the leading role.

Thursday’s pilgrimage and the previous event passed largely peacefully, but insurgent attacks are still common, raising doubts about the Iraqi security forces’ ability to stand alone.