The attack took place in Shura, 50 km (35 miles) south of Mosul, Iraq's third largest city and capital of the Sunni-dominated Nineveh province, where al Qaeda has been regrouping.

The gunmen showered the checkpoint with a hail of bullets from several directions. An ambulance that rushed to the scene was hit by a bomb that had been planted on the road beforehand, wounding the driver and a nurse, police said.

Sunni Islamist militants have been regaining momentum in their insurgency against the Shi'ite-led government in recent months, striking on an almost daily basis.

Iraq's steadily deteriorating security was highlighted by a mass jailbreak near the capital on Sunday when around 500 convicts, including senior al Qaeda operatives, escaped after militants attacked two prisons.

The Islamic State of Iraq and the Levant, which was formed through a merger between al Qaeda's Syrian and Iraqi branches, claimed responsibility for the raids and said it had freed its jailed comrades after months of preparation.

One security official told Reuters on Tuesday that some of the escaped inmates were heading to Syria to join the ranks of the mainly Sunni rebels fighting to overthrow President Bashar al-Assad, whose Alawite sect derives from Shi'ite Islam.

Sectarian tensions across the region have been inflamed by the Syrian civil war, which has also drawn in Shi'ite fighters from Iraq.

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

(Reporting by Ziad al-Sinjary; Writing by Isabel Coles; Editing by Angus MacSwan)