Laurent Fabius spoke a day after the U.N. Security Council called for "a thorough, impartial and prompt investigation" of the latest allegations against the regime, in a statement that diplomats say was watered down by objections from Syria allies Russia and China.
Opposition forces and activists have said that at least 136 people, including many children, were killed in the Wednesday attack in which most bodies bore no sign of wounds. The United States, Britain, France and others have demanded that a team of U.N. experts already in Syria be granted immediate access to investigate the site.
Speaking on RMC radio, Fabius did not make clear how the use of chemical weapons by the Syrian regime might be proven. But if there is proof of a chemical weapons attack by the regime, "we need a reaction by the international community .... a reaction of force," he said.
Fabius excluded boots on the ground as an option, "but a reaction that can take a form, I don't want to be more precise, of force."
France has been at the forefront of efforts to rid Syria of Assad. It was the first to back the opposition Syrian National Coalition and appoint a coalition ambassador in Paris. With Britain, it pressed for the lifting of European arms sanctions, though once they were lifted pulled back and said it is supplying non-lethal military equipment plus humanitarian aid.
Fabius said he spoke at length with the head of the coalition chief, Ahmad al-Jarba, who "confirmed absolutely" that the regime was behind the chemical attack. The Syrian government has adamantly denied using chemical weapons in an artillery barrage targeting suburbs east of Damascus.
The attack coincided with the visit to Syria by a 20-member U.N. chemical weapons team which only has a mandate to investigate three previous allegations of chemical weapons use. Without a mandate, which needs Syria's approval, the investigators would not be able to visit the site of the attack.
Russian Foreign Ministry spokesman Alexander Lukashevich reiterated his claims that the accusations against Assad could be a bid to get the Security Council to stand by the opposition, and to undermine efforts to resolve the conflict by convening a peace conference in Geneva.
"This all smacks of an attempt, at any cost, to establish grounds for forwarding the demands of the enemies of the regime" to the Security Council, he said, pointing to the timing of "this crime near Damascus" and the arrival of the U.N. team as evidence of "premeditated provocation"
He said Wednesday that Russia's information indicated that the rocket that could have carried the toxic chemical had been launched from positions in a Damascus suburb occupied by opposition forces.
The Turkish and German foreign ministers, while underlining demands for the regime to allow U.N. inspectors to investigate, were vague about the consequences if it doesn't - beyond renewing calls for sanctions.
"Several red lines have been crossed - if sanctions are not imposed immediately, then we will lose our power to deter," Turkish Foreign Minister Ahmet Davutoglu said, speaking at a press conference in Berlin.
His German counterpart was more hesitant about an eventual response.
"I am not speculating about what should happen if these reports turn out to be true," Guido Westerwelle said. "These accusations are so serious, so monstrous that it is necessary to enable a real examination before talking or speculating about consequences."
Davutoglu, speaking through an interpreter, said the allegations "must be cleared up in the most objective way." He said he'd spoken to U.N. Secretary General Ban Ki-moon and told him that "the U.N. must not behave hesitantly anymore; sanctions must now be imposed."
Turkish Deputy Prime Minister Bekir Bozdag insisted it was clear that the Assad regime had used chemical weapons.
"It is clear that chemical weapons were used. It is clear that in Syria, only the Assad administration is in possession of this weapon," Bozdag said. "The whole world knows who has what amount of weapons, where these weapons are and where they go to. This is clear as day. Everyone knows who used the chemical weapons."
Suzan Fraser in Ankara, Geir Moulson in Berlin, Laura Mills in Moscow and Monika Scislowska in Warsaw contributed to this report.