Gunmen ambushed a bus carrying Iraqis working for the US military, killing 17 civilians and wounding 13 in Tikrit today, while a car bomb and a gun attack killed four members of the Iraqi security forces elsewhere in northern Iraq.

Some American troops due to leave Iraq soon might have to stay longer due to the current spate of violence, US defence secretary Donald Rumsfeld has said.

Iraqi agents working as security guards at the UN headquarters in Baghdad may have helped the bombers who killed 23 people earlier this week, a US official said today.