There has been an explosion in Karbala south of the Iraqi capital Baghdad. Unconfirmed reports say three people have been killed and 12 injured. Explosions and mortar fire were also heard near the US HQ in Baghdad. US military officials say it is not known if the Baghdad blasts have caused any damage or injuries. Witnesses heard a loud blast coming from the west bank of the Tigris River, where the US-led administration has its headquarters in a palace complex, Reuters reported. In Karbala, the blast on a busy street happened close to the gold-domed Imam Hussein shrine - one of the holiest sites in Shia Islam, Associated Press reported. The bomb was apparently placed in a parked car. Karbala was rocked last month by clashes between supporters of rival Shia factions. Also in October, gun battles broke out between US troops and backers of a maverick Shia preacher, leaving three American soldiers dead. **PHOTO CAPTION*** Polish soldiers stand guard in front of the Imam Abbas shrine in the Shiite Muslim holy city of Karbala. Bulgarian and Polish coalition soldiers have been deployed in Karbala. (AFP/File/Karim Sahib)