France has raised the prospect of the use of force against the Syrian government if allegations of its use of chemical weapons are proved.
The French foreign minister, Laurent Fabius, said on Thursday that if the regime was shown to be responsible for the massacre, "we need a reaction by the international community … a reaction of force". He ruled out the deployment of foreign ground troops but added "a reaction that can take a form, I don't want to be more precise, of force" – raising the possibility of air strikes by western powers.
Hundreds of civilians are known to have died in the attack on Ghouta, a rebel-held area in the Damascus suburbs, and the death toll continues to rise as more bodies are found.
In London, a Foreign Office spokeswoman said the UK would not rule out any option in its response to the latest massacre.
"Yesterday saw a serious escalation in the crisis in Syria," the spokeswoman said. "Our immediate priority is to verify the facts and ensure the UN team is granted access to investigate these latest reports. We believe a political solution is the best way to end the bloodshed. However, the prime minister and foreign secretary have said many times we cannot rule out any option, in accordance with international law, that might save innocent lives in Syria."
The Turkish foreign minister, Ahmet Davutoğlu, declared that the apparent gas attack crossed "all red lines" and criticised international inaction, as well as the UN security council which he said "has not even been able to take a decision".
On Wednesday, the security council expressed "strong concern" and called for more "clarity" on the use of chemical weapons, but Russia and China insisted on the watering down of a tougher approach backed by the US, UK, France and 32 other governments that called on the UN investigative team already in Damascus to be allowed immediate access to the site of the attack, and to be granted greater latitude by the Syrian government to carry out their enquiries.
Bashar al-Assad's regime had previously allowed the UN team – led by a Swedish scientist, Åke Sellström – into the country but limited its investigation into chemical weapons use to three sites.
Moscow and Beijing have consistently backed Assad throughout the civil war, and the Russian foreign ministry on Wednesday accused rebels of staging the massacre to trigger intervention. China issued a statement saying it opposed the use of chemical weapons but called for the UN team to "fully consult with the Syrian government and maintain an objective, impartial and professional stance, to ascertain what really happened".
The opposition Syrian National Coalition has said the Ghouta attack was just the latest in a series of chemical weapons atrocities and joined international calls for Sellström's investigators to be given immediate access. The area was reported to be still under sustained Syrian army bombardment on Thursday.
"We cannot accept massacres, particularly involving the use of these extremely dangerous weapons – we're talking about mustard gas, sarin, things that remind us of the horrors of war," Fabius said. "The last time gas of this type was used on a massive scale was during the Iraq war, by Saddam Hussein."
France has frequently taken the lead in condemning Assad for atrocities but has also stressed that its response would be in concert with its allies, leading to criticism of inaction within France. The headline of Le Monde's editorial on Thursday was: "Indignation is not enough".