Six French citizens have had their passports confiscated and 40 more will be banned from leaving the country after allegedly planning to travel to Syria and Iraq, interior minister Bernard Cazeneuve says.

It is the first time the measure has been used in France following its introduction as part of a raft of new counter-terrorism laws in November.

"If French people go commit attacks in Iraq or in Syria, on their return they will present an even greater danger of carrying out large-scale terrorist attacks on the national territory," Mr Cazeneuve said.

"There are currently six administrative bans on leaving the country that have already been signed, and around 40 that are being prepared."

A security source earlier said the six French were about to travel to Syria "imminently".

Their passports and identity cards have been confiscated for six months, after which the order can be renewed.

Mr Cazeneuve highlighted his ministry's efforts to set up a warning system through which friends and family can alert authorities about potential jihadist cases.

He said the ministry had been alerted to over 1,000 cases and that "several dozen" planned trips to Syria and Iraq had been prevented as a result.

Mr Cazeneuve has also been in California in recent days, meeting with major internet firms in a bid to improve information-sharing about online jihadist networks.

They are due to meet again in Paris in early April, he said.

Some 1,400 people living in France have either joined the jihadist cause in Syria and Iraq or are planning to do so, prime minister Manuel Valls said last month.

AFP