The warning, from Prime Minister Manuel Valls, followed the weekend arrest of Medhi Nemmouche, a French militant suspected of carrying out last week's Brussels Jewish Museum killings after spending a year fighting in Syria.

Valls told BFMTV that the numbers of French citizens or residents who have taken part in the fighting in Syria or were planning to do so now exceeds 800, including some 30 who have died in the conflict.

"We have never before faced a challenge of this kind," Valls said. "It is without any doubt the most serious threat we face.

"We have to ensure the surveillance of hundreds and hundreds of French or European individuals who are today fighting in Syria."

Nemmouche was arrested in Marseille on Friday carrying weapons similar to those used in the Brussels attack and a camera containing a short film in which he appears to claim responsibility for the assassin-style shooting.

Nemmouche is thought to be the first veteran of the Syrian conflict to have carried out an attack of this kind in Europe.

Many terrorism experts have warned it is inevitable that there will be others as battle-hardened, traumatized and weapons-trained militants make their way back to France and countries such as Belgium, Denmark and Britain, which have also tracked hundreds of their citizens on the war trail to Syria.

France was one of the open supporters of the war in Syria, providing widespread support to the militant groups for continuing their war in Syria.

But Paris seems to have been toning down its support to the military confrontation in the Arab country, with President  Francois Hollande calling for political solution following a meeting with head of the so-called Syrian National Coalition (SNC), Ahmad al-Jarba, late August in Paris.

Syria sank into war in March 2011 when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.

The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.

SHI/SHI