"There are 3,000 Europeans waging jihad in Iraq and Syria today. When you do a projection for the months to come, there could be 5,000 before summer and 10,000 before the end of the year," French Prime Minister Manuel Valls said.
"Do you realise the threat that this represents?" he asked in an interview with French channel iTele.
"There have already been nearly 90 French people who have died out there with a weapon in their hand, fighting against our own values," Valls said.
France, along with Belgium, has seen the largest numbers of volunteers leaving to join the Islamic State terorist group, which has seized large parts of Syria and Iraq. Officials said about 350 Belgians have traveled to fight in Syria, the highest number in Europe relative to population.
Last month, France seized passports from six of its citizens and banned 40 more from travelling abroad after they were allegedly planning to travel to Syria and Iraq
It was the first time the measure had been used in France following its introduction as part of a raft of new counter-terrorism laws in November.
"We have to face a particularly high threat level in France, in Europe and in other countries," said Valls.
Britain and Spain are also considering removing the passports and citizenship of those convicted of traveling to join terror groups.
Foreign fighters are often featured in Islamic State propaganda and recruiting videos.