A car bomb killed 33 people and wounded at least another 25 in a market in the Iraqi town of Balad north of Baghdad today.
A US military spokesman said the blast occurred near an Iraqi army checkpoint. The violence came as the US military said impatience with slow improvements to basic services like electricity and water could reverse recent security gains.
Further details of the attack have yet to emerge but elsewhere ten suspected al Qaeda insurgents were killed in clashes with local security volunteers in northern Iraq today, the US said. Five of the volunteers were also killed.
US Defence Secretary Robert Gates arrived in Iraq today to discuss troop levels in the light of improving security and to prepare talks on a pact that will define future relations between Washington and Baghdad.
With Iraq's Shi'ite-led government deadlocked on the 2008 budget and other major laws, US military spokesman Rear Admiral Greg Smith said that Iraq needed to take advantage of security gains to improve the lives of Sunni Arabs.
"What's necessary to come behind security are essential services ... part of that is through the central government's distribution of funds into the provinces," Rear Adm Smith told reporters.
Millions of Baghdad residents still receive only fitful supplies of water and electricity after sectarian fighting and a Sunni Arab-led insurgency killed tens of thousands of Iraqis and devastated infrastructure.
Minority Sunni Arabs, dominant under Saddam Hussein, have long complained about being marginalised since Saddam's fall. Washington has set a series of "benchmark" laws it says are important to draw Sunnis into politics and away from insurgency.
But Prime Minister Nuri al-Maliki's government has been unable to make serious headway on many laws, such as allocating provincial powers and a measure on the oil industry, after the main Sunni bloc walked out of cabinet last August.
Attacks are down by 60 per cent across Iraq since last June, with improved security credited in part to the growth of neighbourhood police units pioneered by Sunni tribal sheikhs in Anbar who turned against al Qaeda in late 2006.
Also today, a US soldier was convicted by a court martial of murdering an unarmed Iraqi man last May and then planting a gun on his body.