Christian and Muslim leaders have condemned Sunday's wave of bomb attacks at churches that killed 11 people.
These were the first major attacks on Christian churches
Pope John Paul II said he deplored the "unjust aggressions" and declared solidarity with Iraqi Catholics.
Iraq's top Shia Muslim leader, Grand Ayatollah Ali Sistani, said the explosions "targeted Iraq's unity, stability and independence".
Reports said a previously unknown group had claimed responsibility for the attacks in an Islamic website message.
Car bombs exploded outside four churches in Baghdad and one in Mosul, during Sunday evening services.
The Committee of Planning and Follow-up in Iraq said in an online statement that its members exploded the cars in front of the churches and warned of further attacks.
The claim could not be verified.
The first blast happened outside an Armenian church in Baghdad. Three other churches in the city were hit within minutes, including a Syrian Catholic church.
The Baghdad explosions killed 10 people and injured dozens.
A fifth church was attacked in the northern city of Mosul, 350km (220 miles) north of Baghdad. Police said one person died and 11 people were injured.
Iraqi police found a sixth bomb outside a Baghdad church and managed to disarm it, the US military said in a statement.
On Monday, the Pope sent a message of condolence to Catholic patriarch of Iraq, Emmanuel III Delly.
The Pope "firmly deplored the unjust aggressions against those whose only aim is to collaborate for peace and reconciliation in the country", the Vatican said.
A statement from Ayatollah Sistani's office said the attacks were "terrible crimes" and urged the government and the people to work together to end the attacks against Iraqis.
"We stress the need to respect the rights of Christians in Iraq and those of other religious faiths and their right to live in their home, Iraq, peacefully," the statement said.
The Russian Orthodox Church said the attacks were an attempt to spark a religious conflict.
The BBC's Peter Greste in Baghdad says that until now there have been no significant attacks on Iraq's Christian minority.
However, many Christians had become increasingly concerned about the possibility of violence, following attacks on Christian-owned shops selling alcohol.
Used to number 1 million
Now estimated at 650,000 - about 3% of population
Main communities: Chaldean Catholics, Assyrian
Other rites include: Armenian, Syrian Orthodox, Syrian Catholic, Anglican
Mainly live in Kirkuk, Irbil, Mosul, Baghdad
An Iraqi interior ministry spokesman described Christians as one of Iraq's most respected groups.
But he also said the attackers may have been trying to antagonise the multinational forces in Iraq, who are from mostly Christian countries.
Iraq's national security adviser, Mowaffaq al-Rubaie, has blamed the attacks on Abu Musab al-Zarqawi, an Islamic militant suspected of links with al-Qaeda.
"Zarqawi and his extremists are basically trying to drive a wedge between Muslims and Christians in Iraq. It's clear they want to drive Christians out of the country," Mr Rubaie told Reuters news agency.
The Jordanian-born militant is blamed for a string of suicide bombings in Iraq. His group has also claimed responsibility for the beheadings of an American and a South Korean hostage.
Also on Sunday, a roadside bomb near the town of Samarra killed two American soldiers and injured a third, the US military said.