Christians and Muslims must remain "united together in prayer," the pope said.

"The past night has struck us and shaken us deeply," Bishop Gebhard Furst told attendees at Rottenburg's Cathedral of St. Martin.

The struggle to liberate Mosul from ISIS militants has entered a third month of often savage combat.

The woman's killing in a public square was the cruelest moment in the descent of this once proud city.

Those killed were mostly women and children. No group has so far claimed responsibility for the attack on Sunday.

Emotions ran high when the church bell tolled for the first time in more than two years.