One Iraqi civilian was killed and 11 were wounded on Friday when a homemade bomb went off in eastern Baghdad, a police source said, the dpa reported.
"An explosive charge was detonated while a small bus was operating in Ghadier region in eastern Baghdad, killing a civilian and injuring 11," the source, who asked for anonymity, told independent Voices of Iraq (VOI) news agency without giving further details.
Meanwhile, a US soldier was killed on Thursday near Fallujah, about 70 kilometres west of Baghdad, the US army announced on Friday.
The incident raises the number of US troops killed in Iraq in August to 15.
A total ban on the movement of cars was imposed in Karbala Friday as part of tight security measures in the holy southern Iraqi city in the run-up to Sunday's pilgrimage to commemorate the birth of the twelfth Imam revered by Shiites.
"A total car ban is imposed on Karbala from today till the end of pilgrimage on Sunday," a spokesman for the security forces in Karbala told VOI.
"Security forces were deployed on all entrances of the city to ensure total security during the commemoration of the birth of Imam al-Mahdi."
Millions of Shiite pilgrims from Iraq and other neighbouring Arab countries are expected to pour into the city 110 kilometres south- west of Baghdad.
Several thousands have reportedly walked to the city since last Monday and the flood of pilgrims is expected to peak during Saturday and Sunday.
Shiite pilgrims have been a constant target for extremist Sunni Muslim groups operating in Iraq, mainly members of the al-Qaeda terrorist network, during their annual celebrations of religious events.
On Thursday, at least 20 people were killed and around 100 injured in the Iskandariya district 40 kilometres south of Baghdad when two women suicide bombers wearing explosive belts blew themselves up among pilgrims on their way to Karbala.
US and Iraqi authorities say al-Qaeda militants have increasingly relied on women to conduct suicide attacks as they can evade security searches.
In other news, Iraqi President Jalal Talabani underwent heart surgery in the US, the Iraqi news agency Aswat al-Iraq reported late Thursday, Talabani's office in Baghdad said.
The 74-year-old president was treated in a hospital in Rochester in the US state of Minnesota.
He had been released from hospital but was still recovering in the US, the report said.
Talabani, an ethnic Kurd, heads the three-person Iraqi presidency.