France has seen 271 militants return from war zones in Iraq and Syria and all of them are subject to investigation by public prosecutors, the country's interior minister said in a newspaper interview.

Some 700 French nationals are estimated to have fought in Daesh ranks in Iraq and Syria, and like other European countries France has been wrestling with how to handle the flow of so-called returnees.

The number of terrorist militants to have returned to France included not only 217 adults but 54 minors as well, with some of them currently in detention, Gerard Collomb said in an interview with Le Journal du Dimanche.

Asked how many militants who held French citizenship had been killed in Iraq and Syria, Collomb told the Sunday newspaper that it was difficult to corroborate information.

The head of France's special forces said in June that his units were directly involved in street battles in the Iraqi city of Mosul but denied they were specifically targeting French-born militants fighting for Daesh.

France has participated in a U.S.-led coalition battling Daesh in Iraq, and it also intervened in Mali to push back a fundamentalist rebellion in the west African state.

French military interventions overseas have exposed it to attack by terrorist militants at home. Gunmen and suicide bombers brutally killed 130 people in and around Paris in November 2015 and over 100 were killed in other terrorist attacks in France in the past two-and-a-half years.

Since the first wave of terror attacks in January 2015, a total of 239 people have been killed.

Collomb said the threat of militant attacks was "very high", citing two incidents targeting police on Paris' Champs-Élysées and seven foiled plots this year alone.

An increasing number of people were being flagged under a preventative monitoring system for radicalized behavior, with more than 18,500 people reported, he said.

French police arrested an intruder brandishing a knife at the Eiffel Tower on Saturday night which led the monument to be evacuated, sources said.

The man, who is aged around 19, forced his way past security guards and then shouted "Allahu Akbar" ("God is Greatest"), a legal source told AFP.

Armed soldiers who are permanently stationed at the Eiffel Tower ordered him to drop his weapon, which he did without attacking anyone, the source said, asking not to be named.

The man, who was apparently alone, was "very quickly overpowered and arrested" and nobody was hurt, a statement from the company that runs the monument said.

After the arrest, police at the site checked the venue and called for it to be evacuated at around 00:30, 15 minutes before the Eiffel Tower normally closes, the statement added.

The legal source said the man with the knife, a Frenchman born in the West African state of Mauritania, had been discharged from a psychiatric hospital in July.

The monument reopened on Sunday morning as usual at 09:00 am.

France has been subject to state of emergency legislation, giving police extended powers, since the November 2015 attacks, and the government plans to incorporate some of these measures into ordinary law through a counter-terrorism bill to be put before parliament in the coming months.