The blast in Muqdadiya, 80 km (50 miles) northeast of Baghdad, was the latest in a series of attacks testing the mettle of Iraqi security forces as US troops prepare to end combat operations at the end of August before a full withdrawal next year.

Four of the dead were Iranian pilgrims, who have flocked to Iraq’s Shiite religious sites in the hundreds of thousands since the US-led invasion removed Sunni dictator Saddam Hussein in 2003. Saddam banned Shiite rites and fought an eight-year war with Shiite-ruled Iran in the 1980s.

All of the wounded were Iranian, said a police source and a source in the security operations center of Diyala province, where a volatile ethnic and religious mix of Sunnis, Shiites and Kurds has helped sustain a stubborn insurgency.

Overall violence in Iraq has fallen sharply since the height of sectarian warfare between once-dominant Sunnis and majority Shiites in 2006 and 2007, but attacks have risen since an inconclusive election in March and before the end of US combat operations this month.

No coalition won a majority in the March parliamentary election. Shiite, Sunni and Kurdish coalitions have been jockeying for position in the new government ever since.

Iraqi and US military officials say insurgents are seeking to exploit the tension caused by the failure of political factions to agree on the country’s next government.

Prime Minister Nuri Al-Maliki’s Shiite-led bloc is locked in a tussle with the vote leader, the Sunni-backed Iraqiya alliance of former Prime Minister Iyad Allawi, in the race to head a new government. Five months after the vote, neither side has been able to assemble the necessary majority.

Insurgents linked to Al-Qaeda appear to be challenging Iraqi police and soldiers as the United States prepares to cut its troop numbers to 50,000 by Sept. 1, compared with a peak of about 170,000 three years ago.

The number of civilians killed in bomb blasts and other attacks soared in July to 396 from 204 the month before, according to Iraqi government figures.