Terrorists want to "turn everyone against everyone" and the danger of returning French fighters from the Middle East will remain a long term threat, a senior French foreign ministry official warned months ago.

In an interview with visiting Australian journalists in Paris in June – five months before terrorist attacks killed at least 129 people in the French capital on Friday – the official detailed how France perceived the threat of terrorism and how authorities were tackling it.

Some 1700 French citizens were then known to have tried to join the so-called Islamic State in Syria and Iraq, the most among European nations. Of that number, about 90 were believed to have been killed and another 150 prosecuted, the official said.

Given France's population of five million Muslims and the effectiveness of IS propaganda, it was a "daily miracle" there had not been a repeat of the January attack on the satirical French weekly newspaper Charlie Hebdo that left 11 people dead in Paris.

"We think, unfortunately, it's not going to stop," the official said. "We're going to have to live with it for years."

"They would like to turn everyone against everyone," he said. "The objective of terrorists is to create a civil war."

Meeting the official required passing multiple screenings inside the Foreign Ministry building, a level of scrutiny unmatched at other interviews during the two-week visit to France and Germany hosted by the two countries' governments.

France on the frontline

France has led Europe's military efforts to counter Islamic militants, such as dispatching special forces to Mali in 2013 and becoming the first European nation to join the US air strikes on IS in Iraq. In September, France became the only European country to extend the airstrikes to Syria.

After the Mali intervention, authorities worried that Muslims – many of whom are from the sub-Saharan nation – would protest strongly, the official said. "We didn't observe that at all."

Syria, though, remains the potent drawcard for those trying to radicalise citizens from France or elsewhere. Combined with social media, the propaganda has been much more effective than during other conflicts.

For instance, during 2001-2012, only about 60-70 French citizens were known to have journeyed to fight in Pakistan and Afghanistan, the official said.

And unlike the 1990s, identifying potential radicals within mosques had become "a nightmare for intelligent services".

The would-be fighters were often "isolated individuals" who might be radicalised within just one month. About 20 to 30 per cent of the French citizens seeking to fight for IS were converts to Islam, he said.

Few of the identified radicals were dual citizens and - at the time of the June meeting - France was not overly worried that terrorists might join the many migrants streaming into Europe from North Africa and the Middle East. "It's not the biggest concern," the official said.

The migration issue has since magnified with hundreds of thousands of people fleeing to Europe. Early reports suggest that one of Friday's dead terrorists may have had a Syrian passport, although authorities are continuing investigations.

Tracking French – or other citizens – who head to Turkey and then cross the border to join IS in Syria and Iraq is difficult. For instance, about one million French people visit Turkey each year and don't require a visa.

Combating the threat involved the use of psychologists and social workers to try to stop people leaving France in the first place. A toll-free number had gathered more than 2000 phone calls from friends and families worried about the behaviour of individuals, many of whom had been prevented from going.

Likewise, intelligence services needed to work closely with customs and immigration, and the courts to intercept returnees from IS, the official said.

The ongoing civil wars in Syria and Iraq were likely to continue to foster extremism.

President Bashar al-Assad was "not part of the solution", the official said, noting that people in some areas were fleeing to IS-controlled areas because they were "perceived less horrible than the government".

Iraq was another "key to the solution", and required the Shia-dominated government in Baghdad being more inclusion to the Sunni minority.

Those efforts, though, were "not very successful for the time being", the official said during the June meeting.