The collapse of ISIS in Iraq and Syria could end up increasing the risk of terror attacks on the West, the country's anti-terror prosecutor has warned.

ISIS has suffered a series of military setbacks after losing territory in Iraq in recent months while one of its most prominent leaders, Abu Muhammad al-Adnani, was killed in an air strike in Syria in August.

But with their forces under pressure, more French jihadis and their families could chose to return home, according to Paris prosecutor Francois Molins.

The collapse of ISIS in Iraq and Syria could end up increasing the risk of terror attacks on the West, the country's anti-terror prosecutor has warned

He told the newspaper Le Monde: 'We see clearly in the history of terrorism that when terrorist organisations are in difficulty on their own turf they look for an opportunity to attack abroad.'

Many of those involved in deadly attacks in France since last year were French born.

It comes as Molins announced harsher prison sentences for returning French jihadis.

Molins, whose office is in charge of terrorism investigations, said around 700 people from France are fighting for extremist groups in Iraq and Syria and 'at one moment or another we will face the return of a large number of French fighters and their families.'

He said his office will hand down more severe criminal charges, with possible sentences up to 30 years, in cases that might previously have drawn maximum sentences of 10 years.

Mohamed Lahouaiej Bouhlel, a 31-year-old Tunisian, rammed a 19-tonne truck into people celebrating Bastille Day in Nice, killing 85 people and wounding more than 400

Molins said there were 26 terrorism cases in 2013, while today his office is following 324 cases.

France is still on high alert following a wave of deadly attacks on its on soil.

In July, a pair of ISIS knifemen stormed a church in Normandy before slitting the throat of an elderly priest.

It came just weels after Mohamed Lahouaiej Bouhlel, a 31-year-old Tunisian, rammed a 19-tonne truck into people celebrating Bastille Day in Nice, killing 85 people and wounding more than 400.

ISIS said Bouhlel staged the attack 'in response to calls to target nations of coalition states' fighting the jihadist group.

Last November, coordinated suicide attacks in Paris killed 130 people and wounded more than 350 at a concert hall, cafes and the national stadium

In July, a pair of ISIS knifemen stormed a church in Normandy before slitting the throat of elderly priest Jacques Hamel

A month earlier a man claiming allegiance to ISIS stabbed a police officer to death before slitting his partner's throat in front of their young son at their home in Magnanville, west of Paris.

Last November, coordinated suicide attacks in Paris killed 130 people and wounded more than 350 at a concert hall, cafes and the national stadium.

In January last year, Gunman Amedy Coulibaly, claiming allegiance to ISIS, killed a policewoman in a Paris suburb before attacking a Jewish supermarket the next day, where he killed four more people. He was later killed in a police assault.