Valls was responding to claims in German newspaper Bild on Monday, that the US National Security Agency had listened in on a senior-level Al Qaeda teleconference, where plans to target Europe’s rail system were discussed.

“Our [intelligence] services have produced not a single piece of evidence which lends credibility to these [claims],” Valls told French radio RTL.

“That’s not to say that our vigilance is not total,” he added.

That caution regarding Monday’s NSA claims was echoed by an unnamed source within the French intelligence services, who told France's TF1 TV there had been no threat to “specifically target France.”

“French intelligence agents haven’t received any threat to specifically target France, but there are some worrying elements for northern Europe as a whole,” the source added.

Valls, for his part, reiterated that France was currently on a “Level Red” terrorist alert – the second-highest available threat level.

“Trains, stations and airports, especially during this period of people returning from summer holidays, have all been the subject of regular surveillance measures and security operations,” he said.

The minister warned, however, that the terrorist threat to France was still very real.

“The terrorist threat is present. We’re not minimising that. It doesn’t only come from organized outside groups like Al Qaeda…but also from within, due to the radicalisation of a certain number of individuals and groups who could act,” he added.

France has only once raised its terror alert to the highest level, scarlet – that was in March 2012, when Islamist gunman Mohamed Merah went on a killing spree in and around the southern city of Toulouse.

In January, the alert was raised to “reinforced red” when leaders of militant Islamist groups under attack in Mali warned that France had "opened the doors of hell" by unleashing its warplanes and called on fellow extremists to hit back on French soil.

A week later, the Mauritanian spokesman for a terrorist group responsible for a deadly kidnapping at the In Amenas gas plant in Algeria, delivered a chilling warning to the French public.

In a warning deliberately aimed to stir up fear in France, the spokesman, who called himself Joulaybib, said there would be repeats of recent terror attacks carried out on French soil by self-proclaimed Islamist extremists.

“I hope France realizes that there will be dozens of Merahs and Kelkals," Joulaybib said.

Khaled Khelkal was an Algerian terrorist who took part in a series of bomb attacks on the Paris metro in 1995.