BAGHDAD/MOSUL (Reuters) - Car bombs exploded outside at least five Christian churches in Iraq on Sunday, killing more than a dozen people and wounding many more in an apparently coordinated attack timed to coincide with evening prayers.

"We are expecting a huge number of casualties," an Interior Ministry source told Reuters, saying there had been four blasts at churches in Baghdad and two in the northern city of Mosul. Police in Mosul said they knew of just one church attack there.

The Vatican condemned the blasts -- the first attacks on churches during the 15-month insurgency -- echoing concerns among Iraqis that they aimed to inflame religious tensions.

In the deadliest attack, a suicide car bomber drove into the car park of a Chaldean church in southern Baghdad before detonating his vehicle, killing at least 12 people as worshippers left the building, witnesses said.

The U.S. military has warned that guerrillas opposed to the presence of more 160,0000 foreign troops may try to deepen divisions between the country's diverse religious communities in their campaign to destabilize Iraq.

"It is terrible and worrying because it is the first time that Christian churches are being targeted in Iraq," said Vatican deputy spokesman Father Ciro Benedettini.

A U.S. military spokesman said three of the four attacks in Baghdad were known to be suicide car bombings.

An explosion at the Armenian church in Baghdad shattered stained glass windows and hurled chunks of hot metal. Another bomb exploded 15 minutes later at a nearby Assyrian church.

"Worshippers were inside the church and during the service a bomb went off," said Shakib Moussa Jibrail, a Christian.

An ambulance driver told Reuters that two people were killed in the explosion at the Assyrian church and several wounded.

U.S. Colonel Mike Murray of the 1st Cavalry Division said at least 50 people had been wounded at the church, some seriously. Those are terrorist acts against the Iraqi people and against Iraq, and we're going to finish them (the terrorists)," Interior Minister Falah al-Naqib told reporters at the church. In Mosul, officials said at least one person was killed in a blast at a church and 15 wounded.

There are about 800,000 Christians in Iraq, most of them in Baghdad. Several recent attacks have targeted alcohol sellers throughout Iraq, the majority of whom are Christians of either the Assyrian, Chaldean or Armenian denominations.

Christians account for about three percent of the population of Iraq, where attempts to provoke conflict have mainly focused on Sunni Muslims and members of the Shi'ite Muslim majority, who were oppressed by ousted dictator Saddam Hussein.

The U.S. military says a computer disk captured earlier this year contained a letter from Abu Musab al-Zarqawi, a Jordanian militant allied to al Qaeda, calling for attacks on Iraqi Shi'ites to try to spark sectarian conflict in Iraq.

In March, coordinated suicide bombings during a Shi'ite religious ceremony killed more than 170 in Baghdad and Kerbala.