The two men who killed Fr Jacques Hamel in a French church filmed themselves and gave "a sort of sermon" in Arabic before murdering the 85-year-old, a nun who witnessed the atrocity has said.

Fr Hamel was celebrating Mass for three nuns and two parishioners on Tuesday morning in Saint-Etienne-du-Rouvray when the attackers burst in and forced the priest to his knees before slicing his throat, according to authorities and the nun who escaped.

Sister Danielle, speaking on BFM television, described seeing the attackers film themselves and give a sermon in Arabic around the altar before she fled. Paris prosecutor Francois Molins said the other hostages were used as human shields to block police from entering. One 86-year-old parishioner was wounded.