France invoked a never-before-used article of the EU's Lisbon Treaty obliging members of the 28-nation bloc to give "aid and assistance by all the means in their power" to a member country that is "the victim of armed aggression on its territory."
French Defence Minister Jean-Yves Le Drian said EU partners could help "either by taking part in France's operations in Syria or Iraq, or by easing the load or providing support for France in other operations."
President Francois Hollande yesterday vowed to forge a united coalition capable of defeating the jihadists at home and abroad.
The Paris attacks on Friday, claimed by IS, have galvanized international determination to confront the militants.
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Hollande has said the victims came from at least 19 nations, and the international community, led by the US and Russia, must overcome their deep-seated divisions over Syria to destroy IS on its home turf.
US Secretary of State John Kerry flew to France as a gesture of solidarity and met Hollande and Foreign Minister Laurent Fabius today.
Kerry said, "we have to step up our efforts to hit them at the core where they're planning these things and also obviously to do more on borders in terms the movement of people."
Earlier, he told US Embassy staff Friday's attacks were an assault on civilization and common decency.
"This is just raw terror," he said, vowing that the campaign against IS will succeed. "We will end the scourge of Daesh," he said, referring to the group by its Arabic acronym.
Interior Minister Bernard Cazeneuve today said that police carried out 128 police raids overnight, as he conceded that "the majority of those who were involved in this attack were unknown to our services."
Police have seized a Kalashnikov assault rifle, three automatic pistols and a bulletproof vest from a suspected arms dealer with jihadist sympathies, and a rocket launcher and other military-grade gear from his parents' home.
But police have yet to announce the capture of anyone suspected of direct involvement in Friday's slaughter.