Gunmen have kidnapped the archbishop of the Chaldean Catholic Church in the northern Iraqi city of Mosul and killed three of his aides, his church says.
The church's community in Iraq is said to be 550,000-strong
Archbishop Paulos Faraj Rahho was seized as he left a church in the eastern al-Nour district, it added.
Pope Benedict XVI deplored the kidnapping as a "despicable" crime.
Most of Iraq's estimated 700,000 Christians are Chaldeans - Catholics who are autonomous from Rome but recognise the Pope's authority.
Many have been targeted since the 2004 invasion by Sunni extremists groups.
In January, bombs exploded outside three Chaldean and Assyrian churches in Mosul. Several Christian priests have also been kidnapped or killed during the past five years.
Archbishop Rahho had just left the Church of the Holy Spirit in Mosul, where he had been leading afternoon prayers, when his car was ambushed by armed men, a church spokesman said.
The gunmen opened fire on the car, killing his two companions and driver, before kidnapping the archbishop, he added.
Bishop Rabban al-Qas of the nearby city of Irbil said his 65-year-old colleague, who was ordained archbishop of Mosul in 2001, was "in the hands of terrorists".
"But we don't know what physical condition [he is in]," he told the Rome-based Catholic news service, AsiaNews.
"It's a terrible time for our church - pray for us," he added.
The kidnappers have reportedly communicated their demands, but these have not been made public.
The Vatican later issued a statement saying the Pope was saddened by "this despicable act" which "touches the whole of the church in that country".
"The Holy Father asks the universal Church to join in his fervent prayer so that reason and humanity prevails in the kidnappers and Monsignor Rahho is returned to his flock soon," it said.
The incident comes less than year after a Chaldean priest and three sub-deacons were gunned down the same church in Mosul after celebrating Sunday Mass.
The Syrian Catholic archbishop of Mosul, Basile Georges Casmoussa, was kidnapped at gunpoint in 2005, but was released after one day without a ransom having been paid.
There are an estimated 50,000 Christians in the traditionally ethnically and religiously mixed city of Mosul, Iraq's third-largest and a centre of the oil industry.
But a rise in attacks on Christians by Sunni extremist groups such as al-Qaeda in Iraq since the US-led invasion in 2003 has prompted many to leave.
Last June, Pope Benedict told US President George W Bush he was deeply concerned about the plight of Iraq's Christians as a result of the ongoing insurgency.
"Particularly in Iraq, Christian families and communities are feeling increasing pressure from insecurity, aggression and a sense of abandonment," he said.
Originally made up of members of the Nestorian Church, the traditional liturgical language of the Chaldean church is Syriac - a descendent of Aramaic, which is thought to have been spoken by Jesus and his disciples.
The church's community in Iraq is said to be 550,000-strong and its best-known member is Saddam Hussein's former deputy prime minister, Tariq Aziz.
The Chaldean Patriarch, Cardinal Emmanuel III Delly, is based in Baghdad.