US intelligence has warned that Europe faces a new wave of Islamist violence on an even greater scale, concentrated on Paris and Brussels.

“At some point there’s going to be a terrorist diaspora out of Syria like we’ve never seen before,” said James Comey, director of the Federal Bureau of Investigations in New York.

Mr Comey’s remarks reflect growing agreement in US intelligence circles that the group will hit out abroad as it loses ground to the US-led coalition in Syria.

Comey predicted that the military coalition would crush the Islamic State in Syria, but that “through the fingers of that crush are going to come hundreds of really dangerous people and they are going to flow primarily to Western Europe.”

The recent terrorist attacks in Europe have increased pressure for a crackdown from the continent’s governments.

France’s Socialist president Francois Hollande came under fire after it emerged that the man who slashed the throat of an 86-year-old priest during mass on Tuesday was a known would-be jihadist under police surveillance.

After a bloody week in Germany, Chancellor Angela Merkel will interrupt her holiday today to address the nation amid growing pressure from conservative allies to step up security measures and deport failed asylum seekers.

Unity

While religious leaders asked the French president for more security at places of worship, Mr Hollande’s political rivals demanded stronger efforts to track known Islamist sympathisers.

Unlike the show of unity after last November’s attacks in Paris, Nicolas Sarkozy, Mr Hollande’s presumptive presidential challenger next year, has stepped up his attacks since another suspected Islamist drove a truck into a crowd in Nice, killing 84 people.

“All this violence and barbarism has paralysed the French left since January 2015,” Mr Sarkozy told Le Monde. “It has lost its bearings and is clinging to a mindset that is out of touch with reality.”

Video

One of the attackers, 19-year-old Adel Kermiche, was a local man known to intelligence services who twice tried to reach Syria to join Isis. He was facing trial for alleged membership of a terrorist organisation and had been wearing a surveillance foot tag

His tag did not send an alarm because the attack was timed during the four hours when he was allowed leave his parents’ home.

Meanwhile, Pope Francis made his strongest remarks yet on Islamic State violence, putting the group’s campaign in a line stretching back to the two world wars. “People keep talking about security but the real word is war,” he said. “The world is at war because it has lost the peace.”

Pope Francis clarified that he was not referring to a war of religion. “All religions want peace. Others want war.” Additional reporting: New York Times