Syria's opposition demanded on Thursday that United Nations chemical weapons inspectors immediately investigate a besieged rebel-held region hit by an alleged chemical weapons attack that killed hundreds of people a day earlier.

In Paris, French Foreign Minister Laurent Fabius said the international community needed to respond with force if the allegations that the Syrian government was responsible for a chemical attack on civilians proved true, although there was no question of sending troops on the ground.

Syrian President Bashar al-Assad's forces have continued a heavy bombardment of the ring of rebel-held suburbs around the capital, known as the Ghouta region, which activists say will further hinder U.N. investigators from entering the area, only a few kilometers from where the team's Damascus hotel.

"We are asking for this team to go directly, with complete freedom ... to the site of the crimes which took place yesterday," George Sabra, a prominent member of the umbrella opposition's National Coalition, told Reuters.

He said the U.N. Security Council should amend the mission of the team, tasked with investigating a few sites of previous alleged chemical attacks, to give it the right to visit any site.

"But we are doubtful because the mission of these experts is constrained by the Syrian regime, limited to a few areas which it will take them to," he said by telephone.

With Wednesday's death toll estimated between 500 and 1,300, what would be the world's most lethal chemical weapons attack since the 1980s prompted an emergency meeting of the U.N. Security Council in New York. Syrian authorities have denied the army used chemical weapons.