Four US soldiers have been killed and more were injured in two separate bomb attacks north of the Iraqi capital.
Three soldiers were killed and another was injured in a blast late on Wednesday near Balad, 80km (50 miles) north of Baghdad.
The attack happened a few hours after a US soldier was killed and four were wounded in a roadside bomb near Tikrit.
At least 1,980 US troops have been killed in Iraq since the US-led invasion in March 2003.
Twelve US military personnel have been killed since Saturday's largely peaceful referendum on Iraq's new constitution.
The blasts came on the day the former Iraqi leader, Saddam Hussein, appeared in court for the start of his trial on charges of crimes against humanity.
He and seven co-defendants pleaded not guilty to charges of ordering the killing of 148 Shia men in 1982. If convicted, they could face the death penalty.