BAGHDAD, July 22 (Reuters) - Five Iranians were killed in the early hours of Wednesday when gunmen opened fire on a convoy of buses taking pilgrims to Shi'ite holy sites in Iraq, police in restive Diyala province said.

The shooting took place on a highway connecting the Iranian border to Baghdad and southern Iraq, about 45 km (27 miles) northeast of the provincial capital Baquba, the police official said on condition of anonymity.

He said five pilgrims were killed, including one woman, and 32 others were wounded when the last bus in a vehicle convoy came under fire.

Interior Ministry forces have been protecting such convoys on the highway, where at least 57 people, mostly Iranian pilgrims, died in an attack in April inside a roadside restaurant where the group had stopped.

That attack, which took place amid a spate of bombings against mainly Shi'ite targets in April, raised fears that Iraq might slide backwards into the worst of the sectarian bloodletting that has killed tens of thousands since the U.S.-led invasion in 2003.

Security has improved greatly, but the future is by no means assured as Iraq's untested security forces take over from U.S. soldiers in many parts of the country and insurgents demonstrate their resilience in ethnically mixed areas like Diyala.

After the April attack, Iran temporarily closed several border crossing with Iraq for Iranian citizens and convoys through Diyala were rerouted for a time.

Pilgrims from Iran, a fellow majority Shi'ite Muslim nation, have flocked to Iraq by the millions since the fall of Sunni Arab Saddam Hussein, just one facet of the close, complex and often tense relationship between Baghdad and Tehran.

The dead were sent to a hospital in nearby Muqdadiya, police said. (Reporting by Suadad al-Salhy; writing by Missy Ryan)

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