UN Secretary-General Ban Ki-moon has said the alleged chemical weapons attack near the Syrian capital “needs to be investigated without delay”.
He is sending disarmament chief Angela Kane to Damascus to press for an investigation, his spokesman Eduardo del Buey said on Thursday.
Earlier, France’s foreign minister said a “reaction with force” might be needed if the claims were substantiated.
Activists say hundreds were killed in attacks on the Ghouta area of Damascus.
“I can think of no good reason why any party – either government or opposition forces – would decline this opportunity to get to the truth of the matter,” said Ban Ki-moon, speaking the South Korean capital, Seoul, on Friday.
“Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law,” he went on. “Such a crime against humanity should result in serious consequences for the perpetrator,”
The UN has asked the Syrian government to allow a team of UN weapons inspectors already in the country to investigate the latest incident.
But the government, which denied the allegations, has given no indication that it will allow this.
The investigators, who arrived in the city on Sunday and are staying about 15km (10 miles) from the site of the recent attacks, only have a mandate to visit three other sites of alleged chemical weapons use.
These include 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.