IS claimed responsibility for a coordinated onslaught in Paris, as well as the downing of a Russian plane over Egypt's Sinai Peninsula last month.
Still reeling from the Paris carnage that killed at least 129 people, France formally requested EU assistance in its fight against the militant group.
France invoked a never-before-used article of the EU's Lisbon Treaty.
It obliges 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."
EU governments agreed unanimously to support France's request.
EU Foreign Policy Chief Federica Mogherini described the attack as an attack on the whole of Europe.
"France has requested aid and assistance in accordance with Article 42-7. Today the European Union unanimously expressed its strongest full support and readiness to provide all the aid and assistance required and needed".
Ahead of a meeting of defence ministers in Brussels, Italy's Defence Minister Roberta Pinotti said it's important that Europe can show France it's united against terrorism.
"We all support France because we all think the attack on Paris was an attack on the core of Europe. Our children are Europe's children and what happened in Paris was felt as if it had happened in their own capital."
It comes as France announced it carried out fresh raids against the IS stronghold of Raqqa in northern Syria, destroying a command centre and a training facility.
France has been carrying out air raids against IS in Syria since September, but stepped up its raids after the Paris attacks.
Meanwhile, Russia's vowed to continue its fight against IS.
President Vladimir Putin ordered the Russian navy in the Mediterranean to establish contact with its French counterparts and work together "as allies" in a campaign against the militant group in Syria.
Fuelled by news that it was a bomb that downed a Russian passenger plane over Sinai, killing all 224 on board, Mr Putin pledged to step up air strikes in Syria.
"We are not going to wipe tears from our souls and our hearts, this will remain with us forever. But this will not prevent us from finding and punishing the criminals. We should be looking for them, no matter how much time passes. We need to know them by their names, we will find them, wherever they are and we will punish them."
British Prime Minister David Cameron has edged closer to extending military action against Islamic State, describing the group as a direct threat to the country.
He's told the British Parliament it's time to reconsider backing air strikes against extremists in Syria.
"Our allies are asking us to do this and a case for doing this has only grown stronger after the Paris attacks. We can not expect, we should not expect others to carry the burdens and the risks of protecting our country."
Mr Cameron is expected to present parliament by the end of this month with a comprehensive strategy for tackling IS.
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