U.N. Secretary-General Ban Ki-moon says any use of chemical weapons in Syria would amount to a "crime against humanity" that would result in "serious consequences" if the allegations prove to be true.
Mr. Ban made the comments during a visit to Seoul, adding that use of chemical weapons under any circumstance would violate international law.
Syrian opposition leaders and activists accuse the Syrian government of using poison gas in an artillery and rocket assault on rebel-held Damascus suburbs on Wednesday. They say hundreds of civilians were killed and released video of scores of bodies of adults and children lying on the ground without signs of injury. Neither the number of people killed nor the cause of death could be independently confirmed.
Mr. Assad's government has denied using chemical weapons in Wednesday's fighting. It has accused the opposition of using chemical weapons earlier this year near Aleppo. A U.N. team is already on the ground in Syria investigating those claims.
On Thursday in a statement released through his spokesman, Mr. Ban said the world body has formally asked the Syrian government to let U.N. personnel "swiftly investigate" the scene of Wednesday's alleged chemical weapon attack near Damascus.
Mr. Ban said he expects to receive a "positive response" from the government of Syrian President Bashar al-Assad "without delay." The U.N. chief said he also asked Under Secretary General Angela Kane to visit Damascus to follow up on the matter.
In a briefing Thursday, White House spokesman Josh Earnest called on the Syrian government to allow the U.N. inspectors to interview witnesses, collect physical samples and have unfettered access to the areas suspected of being hit by chemical weapons.
France said the international community must respond with force if the Syrian opposition allegations are true. But, French Foreign Minister Laurent Fabius ruled out the use of ground troops.
Turkish Foreign Minister Ahmet Davutoglu also called for international action, saying a "red line" has been crossed in Syria.