Paris and London will call for moving up the date of the next European Union meeting on the Syria arms embargo, and will decide to arm the rebels if the 27-member EU does not give unanimous agreement, he said.
France and Britain ask "the Europeans now to lift the embargo so that the resistance fighters have the possibility of defending themselves," he told France Info radio.
If unanimous EU support for lifting the measure is lacking, the French and British governments will decide to deliver weapons, Fabius said.
France "is a sovereign nation," he added.
The next EU meeting to study the embargo is planned for the end of May, but Fabius said Paris and London want to have the meeting earlier.
"We must move quickly," and "and we along with the British will ask for the meeting to be moved up," he said, not ruling out a gathering before the end of March.
Prime Minister David Cameron said Tuesday that Britain would consider ignoring an EU arms ban and supplying weapons to Syrian rebels if it would help topple President Bashar al-Assad.
The EU last month amended its embargo to allow member nations to supply "non-lethal" equipment and training to the opposition but stopped short of lifting the embargo entirely.
"We cannot accept the current disequilibrium with Iran and Russia supplying arms to Assad on the one hand and the opposition unable to defend itself on the other," Fabius said.
French officials, speaking on condition of anonymity, said Paris was considering providing the rebels with ground-to-air missiles to retaliate against air strikes by government troops.
"Lifting the embargo is the only means of moving things on a political level," Fabius said, hinting at pressure on the Damascus regime and Moscow.
Syria's main opposition National Coalition welcomed the announcement.
"We consider it a step in the right direction... Assad will not accept a political solution until he realises he is faced with an (armed) force that will defeat him," spokesman Walid al-Bunni told AFP.
Syria's state news agency SANA said Paris and London's intention to provide weapons to "terrorist groups" was in "flagrant violation" of the principles of international law.
Foreign Minister Sergei Lavrov of Russia - the regime's biggest arms supplier - had made similar comments on Wednesday.