France's foreign minister has said a "reaction with force" could be needed if Syria is proved to have used chemical weapons against civilians.
Laurent Fabius's comments come a day after Syrian activists said hundreds of people died in such attacks in the Ghouta area of the capital, Damascus.
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.
'Red lines crossed'
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".
"We believe a political solution is the best way to end the bloodshed," said the statement, but added that the UK has "said many times we cannot rule out any option... that might save innocent lives in Syria".
Turkish Foreign Minister Ahmet Davutoglu also stressed the need for an urgent response, and criticised the lack of UN action.
On Wednesday, the UN Security Council failed to agree on a statement condemning the attack at an emergency meeting.
It was blocked by China and Russia, which have repeatedly backed the Syrian government since the crisis began.
Russia is supporting calls for an investigation, however, because it believes the opposition might have carried out the attack itself, as "premeditated provocation" in an attempt to win the backing of the UN.
Opposition activists said that more than 1,000 people were killed after government forces launched rockets with toxic agents into the Damascus suburbs in the Ghouta region early on Wednesday.
The BBC has been unable to independently confirm the death toll.
Activists said Wednesday's attack took place as part of heavy government bombardment in the region surrounding Damascus, with government forces trying to drive out rebel forces. The areas affected included Irbin, Duma and Muadhamiya.
Video footage shows dozens of bodies with no visible signs of injuries, including small children, and survivors being treated in makeshift hospitals, with victims, including many children, having convulsions.
Chemical weapons experts have told the BBC that footage appears genuine and that the injuries shown are consistent with nerve agents.
While it is not clear how many died in the bombardment of the sites and how many deaths were due to any exposure to toxic substances, experts say it would be almost impossible to fake so many dead and injured including children and babies.
Both the rebels and government forces have accused each other of using chemical weapons throughout the 28-month conflict.
Syria is believed to have large undeclared stockpiles of mustard gas and sarin nerve agent.
The government has implied it has chemical weapons, but said they would not be used against civilians.
More than 100,000 people are believed to have been killed during the 28-months of conflict Syria.