The blast occured at 7:45pm (1545 GMT) on Thursday on the outskirts of Baghdad's western Kadhimiyah neighbhourhood, the official said.
Iraq's Shia community has been hit by a surge of attacks in recent weeks in what US officials say is a campaign by the al-Qaeda leader in Iraq to draw them into a sectarian civil war.
Last week, a triple suicide bombing at a Shi'ite mosque in Baghdad killed up to 90 people.
In a separate attack on Thursday, armed men killed the brother of a top Sunni Arab politician in Baghdad on Thursday, members of his Iraqi Islamic Party said.
Mahmoud al-Hashimi, brother of Iraqi Islamic Party leader Tariq al-Hashimi, died on Thursday, according to senior party official Ayad al-Samarei. He refused to give further details.
Al-Hashimi is being tipped as the speaker of the new parliament, which is scheduled to meet on Monday.
Thursday's attacks come at a time of growing sectarian tension which has raised fears of civil war between majority Shias and minority Sunnis.
The Islamic Party is one the country's main Sunni political organisations.
Bahjat al-Samarei, a local Sunni official, said Mahmoud al-Hashimi was driving with a friend in eastern Baghdad when armed men opened fire, killing them both.
Tariq al-Hashimi is a leader of the
mainly Sunni Iraqi Islamic Party
Police confirmed such an attack occurred but could not confirm the victim was al-Hashimi's brother.
Earlier this month, two other prominent Sunni politicians said close relatives had gone missing. One of them blamed al-Qaida in Iraq for trying to frighten them into abandoning their roles in government.
Khalaf al-Ilyan, head of the National Dialogue Council, reported the disappearance of his brother and niece in Ramadi, an anti-government stronghold west of Baghdad.
The other Sunni politician, Saleh al-Mutlaq, said his brother disappeared in Salahuddin province, north of Baghdad.