Paris, Nov 15 (IANS): The terror attacks in Paris that left 129 people dead and over 300 injured aren't "one off" and similar deadly attacks are likely to occur again, an expert on terrorism has said.

Magnus Ranstorp told The Local that the Islamic State went on the offensive in France because "the French have been exercising their military muscle. There has been tough talk, particularly after (the January terrorist attacks on the offices of) Charlie Hebdo".

"France are at war and have been for some time, through the coalition," he said.

The terrorism expert explained that close to 2,000 French citizens have been fighting with the Islamic State. This constitutes a third of Europeans within Isis.

"In some countries it has become more difficult to travel to Syria. In France, however, it (travel) has been accelerating," The Local quoted Ranstorp as saying.

He argued that "there have been 17 publicly revealed attacks or foiled attacks in France since the war began in Syria - including the Charlie Hebdo and kosher supermarket attacks in Paris".

"This is an escalation on this issue. The French have to worry more than most other countries about two fronts - both North Africa and ISIS," Ranstorp said.

"Why did they attack Paris now? IS have been losing territory inside of Syria and to some extent in Iraq. They, therefore, are going on the offensive to make up ground after these losses."

"I’ve been speaking to intelligence agencies in Europe, who say that the moment they turn their attention towards Europe we have a problem," the terrorism expert said.

"It’s not a one-off, it’s going to continue. It may take a while - they’re clamping down," he said.