Christmas carols have been sung in Mosul's Saint Paul cathedral for the first time since the Iraqi city was overrun by Islamic State militants.

The Church of the Immaculate Conception, burned by Islamic State militants, in the town of Qaraqosh, south of Mosul. ( Reuters: Ari Jalal )

It was the first Christmas service held in Iraq's second largest city in four years.

Under IS rule any public celebration of Christian rituals was impossible and many Christians fled the city and persecution.

The IS group took control of Mosul in June 2014. The last extremist fighters were finally pushed out in July.

Security was tight but dozens of worshippers packed into the church for a Christmas Eve mass.

The Patriarch of Iraq's Chaldean Catholic Church, Louis Raphael Sako, led the service.

"My message is to our brothers the Muslims before the Christians," he said.

"I ask them to change their way of thinking, you should know Christianity better.

"In the past Christians were the majority in Iraq, today we are minority but without us Mosul will never be the same."

He expressed hope that Christians who fled persecution in Mosul would be able to return to their homes now Islamic State militants had been driven out.

"They are not going back because their houses are destroyed or burnt and the church is restoring all of the houses," Patriarch Sako said.

"We are hopeful that many many Christians will be back."

Christmas Eve mass is held at Saint Paul cathedral in Mosul, Iraq. ( Twitter: @alsemawee )

Historian and self-styled documenter of Mosul's time under Islamic State rule, Omar Mohammed (or @MosulEye), said:

"For the first time in my lifetime I am seeing people enjoying Santa Claus and Christmas Eve in Mosul."

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ABC/AP