"All the information at our disposal converges to indicate that there was a chemical massacre near Damascus and that the Bashar regime is responsible," Fabius said on a visit to Ramallah in the West Bank.
Opponents of Bashar al-Assad said the president's forces killed 1,300 people when they unleashed chemical weapons east and southwest of Damascus in the attacks on Wednesday.
UN Under Secretary General Angela Kane arrived in the Syrian capital today for talks aimed at establishing the terms of an enquiry into the alleged attacks, an AFP journalist said.
"The information which we have shows that this chemical massacre is of such gravity that it obviously cannot pass without a strong reaction," he added.
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The Syrian government has strongly denied accusations it carried out the attacks, but so far it has not said whether it will let UN inspectors visit the sites.
Fabius arrived early today on a visit to Israel and the Palestinian territories aimed at encouraging recently resumed peace talks, his office said.
He met Palestinian president Mahmud Abbas and prime minister Rami Hamdallah at their headquarters in the West Bank city of Ramallah today.
"This visit will be an opportunity for the minister to encourage these Palestinian and Israeli negotiators to continue direct negotiations in favour of peace," French foreign ministry deputy spokesman Vincent Floreani said.
The French side would also express "determination to support these efforts," he added.
Israeli and Palestinian negotiators formally resumed direct peace talks earlier this month after a hiatus of nearly three years, thanks to an intense bout of shuttle diplomacy by US Secretary of State John Kerry.
"We do not have high expectations of the negotiations so far because we know in advance the official position of the Israeli government," foreign minister Riyad al-Malki said on visit to Quito, Ecuador.