Armed men shot dead 14 Shia men after checking their identity papers at a makeshift roadblock, police said.

The killings took place on the main road on Wednesday near Sulaiman Pek, 160km north of the capital, following clashes inside the town between gunmen and security forces.

"All the victims were Shia tanker drivers who were coming from Baghdad to Kirkuk," Talib Mohammed, the town's mayor, told the Reuters news agency. "Militants blocked their way near Sulaiman Pek, checked their IDs and executed them."

Earlier, gunmen ambushed a minibus in western Tikrit, 150km north of the capital, shooting dead four soldiers who were travelling on the road from Baghdad to Mosul.

Nine policemen were also killed when fighters riding on pick-up trucks opened fire of a checkpoint in Shura, 50km south of Mosul, Iraq's third largest city and capital of the Sunni-majority Nineveh province.

Deteriorating security

Sunni armed groups have stepped up attacks in recent months, invigorated by the civil war in neigbouring Syria, which has inflamed sectarian tensions in Iraq and the region.

The steady deterioration of security in Iraq was highlighted by a mass jailbreak near the capital on Sunday when around 500 convicts, including senior al-Qaeda operatives, escaped after comrades attacked two prisons.

The Islamic State of Iraq, an umbrella organisation of al-Qaeda-linked groups, said it carried out the raids and said it had freed its jailed allies after months of preparation.

Shia fighters from Iraq have joined Syria's conflict on Bashar al-Assad's side, along with Lebanon's Hezbollah.

More than 720 people have been killed in attacks by armed groups in Iraq so far in July, according to the violence monitoring group, Iraq Body Count.