At least nine people were killed on Thursday in a double suicide bombing targeting a mosque for Shiite worshippers in the northwest of the Iraqi capital, police said.

Sunni extremists of the Islamic State (IS) jihadist group claimed the attack.

"Two suicide bombers wearing explosive belts blew themselves up in front of the Rasul al-Azam (mosque) in the Shuala district" of Baghdad, a police official said, adding that more than 20 people were wounded in the blasts.

"The first suicide bomber blew himself up among the worshippers as they left" the mosque, he said.

"The second explosion targeted police forces who had arrived to treat the wounded," causing more fatalities, he said.

Medical sources confirmed the death toll.

IS said in a statement posted on jihadist forums the first attacker blew himself up inside the mosque and other detonated his explosives when security forces arrived at the scene.

The blasts in the mainly Shiite neighbourhood came on the eve of a planned protest that radical Shiite cleric Moqtada al-Sadr has called for in Baghdad and said he would personally attend.

Sadr has called on his supporters to attend protests in the capital and other cities to demand an end to corruption and improved public services.

Tens of thousands of his supporters are expected to take part.