A bomb exploded Sunday in Iraq near a convoy of vehicles transporting U.S. Ambassador Christopher Hill. No one was injured in the attack.

The bombing occurred in Dhi Qar province in southern Iraq. The newspaper, "USA Today" quoted Hill as saying he heard a bang and drove through a cloud of smoke.

Iraqi security forces stand guard outside one of several Christian churches that were bombed in Baghdad, 12 Jul 2009

Earlier in the day, a series of bombings at Christian churches in Baghdad killed four people and wounded at least 18.

The bombs went off near churches in central and eastern Baghdad. The fatal attack took place near a church on Palestine Street in the eastern part of the city.

Islamist militants have staged sporadic attacks on Iraq's small Christian minority in recent years. Thousands of Iraqi Christians fled their homes in the northern Iraqi city of Mosul last year following a series of attacks against their community.

In other violence, gunmen killed an Iraqi Christian local official in the northern Iraqi city of Kirkuk Sunday. Police say the head of Kirkuk's audit department, Aziz Rizko was driving when assailants shot him using guns with silencers. The motive for his killing was unclear.

Sectarian tension is high in Kirkuk, an ethnically diverse city that Kurds want to merge with their autonomous region in northern Iraq. Many Arab and Turkmen residents oppose such a move and want to remain under the control of Iraq's central government.

Violence has declined in most of Iraq in recent months, but militants continue to stage attacks in northern Iraq and the capital, Baghdad.

Some information for this report was provided by AP.