The UN has asked Syria to allow UN weapons inspectors already in the country to be allowed to investigate. But there is no sign as yet that Damascus will allow this.
The UN team arrived in the city on Sunday and are staying about 15km (10 miles) from the site of the recent attacks.
But they only have a mandate to visit three sites previously agreed between the UN and the Syrian government, including the northern town of Khan al-Assal, where some 26 people were killed in an alleged chemical attack in March.
The Syrian government has described the latest allegations as “illogical and fabricated”. The Syrian army said opposition forces had made up the claims to divert attention from their recent huge losses.
Heavy shelling continued around Ghouta on Thursday, reports say. A spokesman for Ban Ki-moon, Eduardo del Buey, said on Thursday that the secretary general Ban Ki-moon believed the attacks “need to be investigated without delay”.
Mr Ban was sending his disarmament chief Angela Kane to Damascus to press for an investigation, he said.
Earlier, Mr Fabius told the French BFM TV channel that if the use of chemical weapons was confirmed, “France’s position is that there must be a reaction, a reaction that could take the form of a reaction with force”.
He did not elaborate on whether that meant backing military action, but did rule out the idea of deploying troops inside Syria.
The US state department said it had yet to “conclusively determine” what had taken place in Damascus, but that it was urgently gathering information.
If President Bashar al-Assad’s government was found to be behind a chemical weapons attack it would be “an outrageous and flagrant escalation”, spokeswoman Jen Psaki said.
President Barack Obama warned last year that the use of such weapons would cross a “red line”.
The British Foreign Office said earlier in a statement that the UK and 36 other countries had formally referred the latest allegations to Mr Ban, and called for inspectors “to be granted the necessary access to enable their investigation into these latest allegations as a matter of urgency”.