At least six passengers have been killed as a bomb blew up a minibus leaving the predominantly Shia area of Bab al-Sharki in Baghdad.
The explosion on Sunday took place as Iraq's security forces remained on high alert during the Shia mourning feast of Ashura.
The blast came less than 24 hours after a US Black Hawk military helicopter crashed northeast of Baghdad in Diyala province.
Police say the bomber left the device in a bag on the bus.
Heavy casualties
Two US Marines died in fighting in Iraq's Anbar province on Sunday, a day after 25 US soldiers died in one of the bloodiest days for the US military in several months.
Twelve passengers and crew were killed in the helicopter crash near Baghdad; initial reports had put the figure at 13.
A group called Jaish al Mujaihadeen claimed to have shot down the aircraft.
Five more US soldiers were killed in the Shia holy city of Karbala in clashes with gunmen. A further five were killed in Anbar province in western Iraq, the military said on Sunday.
The bloodshed comes at a critical time for Iraq and the Bush administration, which has proposed to send about 21,500 more US troops to try to halt the sectarian violence.
On Sunday more than 3,000 of the extra US troops arrived in Baghdad.