An interior ministry official said the blast targeted a wedding convoy at dusk in Ur, a mainly Shia residential district on the outskirts of Sadr city, which one day earlier was also the scene of a deadly bombing.

"A terrorist left a car parked on the street corner and, when the bridegroom's family arrived in a convoy, he set off a bomb," he said.

A defence ministry official confirmed the blast, but said the attack was thought to have been a suicide bomb rather than a remote-controlled device.

Baghdad is in the grip of a deadly sectarian conflict between rival Sunni and Shia factions which kills scores of people every day, despite a security operation involving 15,000 American and 40,000 Iraqi troops.

In Tikrit gunmen manning a fake security checkpoint north of Baghdad kidnapped between 30 and 40 mostly Shia bus passengers heading to the northern town of Balad on Tuesday, police said.

An Iraqi official from the Joint Coordination Centre in Tikrit, which oversees US and Iraqi security forces in the area, said the buses were halted by the gang near Tarmiya, 40km north of Baghdad.

They were carrying Shia passengers from the capital to Balad, a town which was recently at the centre of brutal eruption of tit-for-tat sectarian murders between rival Sunni and Shia armed factions, he said.