A suicide bomber killed 25 people and wounded more than 50 in Baghdad on Friday when he detonated his explosives inside a busy cafe near a park popular with families, police and medical sources said.

It was one of the worst attacks in Iraq since suicide bombers hit the same district two months ago, targeting a Shi'ite mosque and killing 29 worshippers during noon prayers.

Friday's bombing took place in al-Qahira, a northern district of the capital that is home to mainly Shi'ite Muslims. Children were among the casualties at the site, which is in an area with cafes and restaurants. The wounded had been taken to four different hospitals, the sources said.

"There was a crowd of people, and the suicide bomber detonated himself right inside it. Most of the people were killed or injured by ball bearings from the device," police officer Ahmed Jassim said.

His patrol heard the blast and arrived at the site to find people lying dead or injured on the ground in pools of blood. Police hesitated to help the wounded because they feared a second bomb, he said.

In a separate attack in central Baghdad, gunmen on motorcycles shot dead four people who were in a car, police and health sources said.

Iraqis have suffered extreme violence for years, but since the start of 2013 the intensity of attacks on civilians has dramatically increased. More than 1,000 Iraqis were killed in attacks in July, the worst monthly toll since 2008.

Since the start of 2013, bomb attacks by mainly Sunni Muslim insurgents have increasingly targeted cafes and other places where families gather, as well as the usual targets of military facilities and checkpoints.

Eighteen months since U.S. troops withdrew, deep-rooted sectarian tensions have been aggravated by the civil war in neighbouring Syria and growing political divisions between Shi'ite, Sunni and ethnic Kurdish factions.