The UN Secretary-General Ban Ki-moon has asked the Syrian government to allow UN inspectors to investigate "without delay" the latest alleged chemical attack in the country's civil war.
Ban said that the use of chemical weapons in Syria would constitute a "crime against humanity", and warned Bashar al-Assad's government of "serious consequences" if their use was proven.
"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," he said.
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Opposition groups have alleged that Assad's government on Wednesday fired rockets with chemical warheads into the Damascus suburbs of Arbeen, Zamalka and Ein Tarma. The attack reportedly left hundreds dead.
"This is a grave challenge to the entire international community - and to our common humanity, especially considering it occurred when the United Nations expert mission is in the country," Ban said.
"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," the UN chief told a diplomatic forum in the South Korean capital, Seoul.
According to a statement on Thursday, Ban had instructed Angela Kane, the UN's high representative on disarmament affairs, to travel to the Syrian capital.
Meanwhile, activists said government warplanes hit areas where the alleged chemical attack took place again on Thursday.
Navi Pillay, the UN high commissioner for human rights, echoed Ban's sentiment and said the gas attack allegations were "exceptionally grave".
"This absolute prohibition applies in all circumstances... it is binding on the government despite it not being party to the 1993 Chemical Weapons Convention. It is also binding on anti-government armed groups."
Syria's government, which has denied using chemical weapons, has offered no public response to the UN calls for its team to inspect the site of the attack.
The administration of US President Barack Obama, meanwhile, said that it was "appalled" by the reports of chemical weapons use.
Obama ordered the US intelligence to urgently investigate claims that Syrian forces launched a chemical attack on civilians, including children, aides said.
Earlier on Thursday, France said that the international community would need to respond forcefully if allegations that the Syrian government was responsible for a chemical attack on civilians proved true.
Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law
"There would have to be reaction with force in Syria from the international community, but there is no question of sending troops on the ground," the Foreign Minister Laurent Fabius told the French television network BFM.
If the UN Security Council could not make a decision, one would have to be taken "in other ways", he said, without elaborating.
"He appears to be hinting at threating some kind of military action in order to put pressure on Syria," said Al Jazeera's Jacky Rowland, reporting from France.
The Syrian National Coalition has said more than 1,300 people had died, while videos and photographs showed scenes of dozens of people foaming at the mouth and of bodies stacked up in morgues.
Foreign governments have demanded immediate access for the UN chemical weapons investigation team to the sites of the alleged attacks.
On Thursday, the Turkish Foreign Minister Ahmet Davutoglu said a "red line was crossed" in Syria and called for international action.
Washington has previously described chemical weapons use as a red line that might prompt it to intervene militarily in Syria.
Source: Al Jazeera and agencies