Fabius also added his voice to calls for a U.N. team currently in Syria to investigate previous claims of chemical weapons use by either side to be given access to the site of the alleged massacre outside Damascus.
If al-Assad's regime "has nothing to reproach itself with," it should let the U.N. weapons inspectors investigate the alleged attack, Fabius said. "If the Syrians refuse, that means they have been caught red-handed," he said.
Turkey urges decisive action
Speaking on a visit to Berlin, Turkish Foreign Minister Ahmet Davutoglu called on the United Nations to act decisively, Turkey's semi-official Anadolu news agency reported.
Davutoglu said "all red lines" have been crossed without the United Nations taking action in Syria and that the body "can't assume an undecisive attitude about chemical weapon attacks" there.
"If the Syrian regime is confident enough, it should allow the U.N. team to investigate (the chemical attack claims) immediately," he added.
Speaking alongside him, German Foreign Minister Guido Westerwelle said the reports from Syria "are serious and, should they be confirmed, outrageous."
He demanded a prompt explanation of the reported chemical weapons use, and also called for the U.N. chemical weapons team to be allowed access.
"I regret that Russia and China have blocked a formal resolution of the Security Council," Westerwelle said.
Al-Assad's government denied the claims on state-run media Wednesday, calling them "completely baseless."
"Everything that has been said is absurd, primitive, illogical and fabricated," said Syrian Information Minister Omran Zoabi on state TV. He said the claim was timed by the opposition to coincide with the U.N team's visit and came as government forces were making gains on all sides against the rebels.
CNN could not immediately verify where or when the videos were recorded, and could not authenticate the number killed or injured.
Israel: World paying lip service
Yuval Steinitz, Israeli Minister of Strategic Affairs, told Israel Radio on Thursday morning that its intelligence assessments indicated that "chemical weapons were used, and they were not used for the first time."
He accused the international community of "paying lip service" when it comes to Syria.
"Nothing practical, significant, has been done in the last two years in order to stop the continuing massacre of civilians carried out by the Assad regime," he said. "I think that the investigation of the United Nations is a joke."
The White House said Wednesday that officials were "working urgently to gather additional information." It called for the U.N. team in Syria to be given immediate access to witnesses and affected individuals, and to be allowed to collect evidence unhindered by the government.
"If the Syrian government has nothing to hide and is truly committed to an impartial and credible investigation of chemical weapons use in Syria, it will facilitate the U.N. team's immediate and unfettered access to this site," the statement said.
But U.S. senator and former presidential candidate John McCain was strongly critical of the administration's handling of the situation in an interview with CNN's "New Day" on Thursday.
He said chemical weapons use was "obvious from the pictures when you see the dead bodies of children and women and others stacked up."