France says the international community must respond with force if opposition allegations that Syrian forces used chemical weapons in an attack near Damascus prove true.
But French Foreign Minister Laurent Fabius on Thursday ruled out the use of ground troops.
Turkish Foreign Minister Ahmet Davutoglu is also urging international action, saying a "red line" has been crossed in Syria.
Activists say Syrian forces launched new bombing attacks Thursday in the eastern Ghouta area, where a day earlier the opposition accused government troops of killing many civilians with chemical weapons.
The Syrian government has denied the chemical weapons allegations. It says the opposition is trying to distract United Nations inspectors who are in the country to investigate government claims that rebels used chemical weapons earlier this year.
The U.N. Security Council held an urgent session Wednesday to discuss the situation in Syria.
U.N. Deputy Secretary-General Jan Eliasson expressed concern about the new allegations and said U.N. officials were discussing gaining access to the sites with the Syrian government.
"This represents, no matter what the conclusions are, a serious escalation with grave humanitarian consequences and human consequences. We very much hope that we will be able to conduct the investigation."
The Arab League and Western powers, including the United States, also urged Syria's government to allow U.N. inspectors to immediately visit the sites.
Russia, a key Syrian ally, accused the opposition of committing a "premeditated provocation" by making claims about mass casualties from a government chemical attack soon after the arrival of the U.N. inspectors.
Syrian opposition reports of the death toll from Wednesday's attacks varied widely. Opposition leader George Sabra of the exiled Syrian National Coalition told a news conference in Istanbul the number of those killed is as high as 1,300. His claim could not be independently verified.
Syrian activists said government troops unleashed an artillery and rocket barrage against several Damascus suburbs, with some of the weapons allegedly containing chemical elements. They posted videos online showing scores of bodies of adults and children laid out on the floor of makeshift clinics with no visible signs of injuries.
The White House said it is "deeply concerned" by the reports and called for those responsible for using chemical weapons to be held accountable. It said the Syrian government must allow U.N. investigators to "examine and collect physical evidence without any interference or manipulation."
The mandate of the U.N. inspection team is limited to establishing whether chemical weapons - including sarin and other toxic nerve agents - were used, not who used them.
The Syrian government also has restricted the mission to investigating several specific incidents, including a March attack in the Aleppo suburb of Khan al-Assal.