Brigadier-General Mohammed al-Wagaa, police chief in the divided northern city of Mosul, said the cleric and his colleagues had been shot dead outside the Holy Spirit church in the Nur district.

"They finished mass ... and the four of them got into the priest's car to drive away. After they had gone about 100m a car cut them off. Four armed men got out and shot them dead," he said.

The Catholic news agency Asianews identified the victims as 31-year-old Father Ragheed Ganni and his three assistants.

Last month, the leaders of Iraq's Christian minority called on the country's beleaguered government to protect their community from attacks by al-Qaeda-inspired Muslim extremists.

In a joint statement, Patriarch Mar Dinka IV of the Assyrian Church of the East and the Chaldaean Catholic Patriarch Emmanuel Delly of Babylon said Baghdad's remaining Christians were facing persecution.

They blamed the so-called Islamic State of Iraq, an alliance of Islamist insurgent groups that serves as an al-Qaeda front, for much of the violence.

"Christians in a number of Iraqi regions, especially those under the control of the so-called Islamic State of Iraq, have faced blackmail, kidnapping and displacement," the May 10 statement said.

Before the US-led invasion of March 2003, there were estimated to be about 800,000 Christians in Iraq, about three per cent of the otherwise largely Muslim population, living mainly in urban centres such as Baghdad.

Although there were some attacks on churches in the immediate aftermath of the overthrow of Saddam Hussein, Iraq's Christians were not especially targeted while rival Sunni and Shiite Muslim factions went to war.

As a relatively wealthy community, however, many Christians fell prey to kidnap and ransom gangs and many - probably more than half - of them have fled the country or moved to the relative safety of Iraqi Kurdistan.

Now there are reports that Salafist groups such as al-Qaeda, fundamentalists who believe Islam can be renewed by returning to the values of the era of the Prophet Mohammed, are targeting Christians on purely sectarian grounds.

The US Commission on International Religious Freedom, an independent bipartisan government agency, last month voiced concern at the deteriorating situation for freedom of religion and belief in Iraq.

Christian communities now face the threat of eradication in their historic homelands in Iraq under pervasive and severe violence and discrimination at the hands of both government and non-government actors, it warned.