A suicide bomber driving a car killed at least 19 people and wounded 48 others on Saturday in an attack claimed by Islamic State on a group of Shi'ite Muslim pilgrims in a southeastern suburb of Baghdad, Iraqi police sources said.

A second explosion near a Shi'ite militia checkpoint in the capital's Dora district killed two and wounded three others, police sources said. There was no immediate claim of responsibility for that blast.

Amaq news agency, which supports Islamic State, said a fighter in the Nahrawan district had driven a truck loaded with three tonnes of explosives into the gathering of Shi'ite pilgrims. They had been heading to Baghdad's Imam Kadhim shrine to commemorate the 8th century death of one of Shi'ite Islam's senior figures.

Security has gradually improved in Baghdad, which was the target of daily bombings a decade ago, but attacks against both the security forces and civilians are still frequent.

The rise of Islamic State, which is fighting government forces for control of swathes of northern and western Iraq, has exacerbated the country's sectarian conflict, mostly between Shi'ites and Sunnis, which emerged after the 2003 U.S.-led invasion.

Also on Saturday Islamic State militants launched an offensive near the northern town of Baiji, which Iraqi forces and Shi'ite militia fighters recaptured months ago along with a large oil refinery devastated by the conflict.

In the first set of attacks on Friday evening near Siniya west of Baiji, Islamic State seized four security checkpoints, according to sources in Salahuddin operations command. The militants used car bombs, killing 11 members of the security forces and wounding 12 others.

From the eastern side of Baiji, the militants attacked security forces near Makhoul, killing three police officers and wounding two others in fighting that continued on Saturday, the sources said.