Three attacks on separate checkpoints north of Baghdad on Thursday killed eight people, including four policemen and three anti-Qaeda militiamen, Iraqi officials said.

Unknown gunmen killed four policemen in an attack on a checkpoint on a road near Tikrit, 160 kilometres (100 miles) north of Baghdad, a Tikrit police officer and a medic from the town's hospital said.

Three anti-Qaeda Sahwa militiamen were killed by a bomb placed in a caravan at a checkpoint near Balad, 70 kilometres (43 miles) north of the Iraqi capital, a police officer and a medic from Balad said.

And gunmen attacked an army checkpoint in Dujail, 60 kilometres (37.5 miles) north of Baghdad, killing a soldier and kidnapping four others, an Iraqi army officer said.

The latest violence comes a day after official figures put the number of people killed in attacks in July at 325, the highest monthly death toll since August 2010.

While violence has decreased compared to its peak in 2006-2007, attacks remain common across Iraq.