Bombings killed more than 40 people in Iraq yesterday in the worst violence since United States combat troops withdrew from urban areas last week, and American forces released five Iranian officials suspected of aiding Shi'ite insurgents.

US officials said they believe the Iranians, detained in northern Iraq in January 2007, had facilitated attacks on American-led forces but handed them over to the Iraqi government at its request. Iraq's Foreign Minister, Hoshyar Zebari, said it was a "good initiative" that could encourage dialogue between Washington and Tehran, which are long-time foes.

The Iranian Embassy said it expected to receive the Iranians, described by their government as diplomats. Washington believes they are members of the Quds Force, part of Iran's powerful Revolutionary Guard Corps.

The carnage within Iraqi borders yesterday was a reminder that insurgents remain intent on destabilising Iraq as the United States shifts its focus to the war in Afghanistan. Attacks are down sharply from past years of war and militants have been driven from many strongholds, but they routinely inflict casualties in Baghdad and northern Iraq, a cauldron of ethnic and sectarian tension.