At least 32 people have died in attacks targeting mainly Shia Muslims across Iraq, increasing concern of a return to full-blown sectarian war.

More than 1,000 people were killed in militant attacks in Iraq in May, according to the United Nations, making it the deadliest month since 2007.

Two car bombs exploded separately in the predominantly Shia southern city of Basra, killing at least five people and wounding ten.

Car bombs also targeted Shia Muslims in Najaf, Nassiriya, Kut, Hilla, Tuz Khurmato and Mahmudiya in southern Baghdad.

Gunmen shot dead six policemen at a checkpoint in Hadhar near the northern city of Mosul.

No group claimed has responsibility for the attacks, but Sunni Islamist insurgents and al-Qaeda's Iraqi wing have increased operations since the beginning of the year as part of a campaign to increase inter-communal tension.