US National Security Adviser H R McMaster says despite denials, public reports show Syrian President Bashar al-Assad used chemical weapons.
Speaking at a major international security conference taking place in Munich on Saturday he added that it was time for the international community to hold the Syrian government accountable.
"Public accounts and photos clearly show that Assad's chemical weapons use is continuing," Mr McMaster said.
"It is time for all nations to hold the Syrian regime and its sponsors accountable for their actions and support the efforts of the Organisation for the Prohibition of Chemical Weapons," he said.
Mr McMaster did not specify which public accounts or pictures he was referring to.
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French President Emmanuel Macron says "France will strike" if chemical weapons are used against civilians in the Syrian conflict in violation of international treaties, but that he had not yet seen proof this was the case.
The Syrian government has repeatedly denied using chemical weapons and said it targets only armed rebels and militants.
In recent weeks, rescue workers, aid groups and the United States have accused Syria of repeatedly using chlorine gas as a weapon against civilians in Ghouta and Idlib.
Earlier this month, Syrian government forces, who are backed by Russia and Iran, bombarded the areas, two of the last major rebel-held parts of Syria.
Diplomatic efforts have made scant progress toward ending a war now approaching its eighth year, which has killed hundreds of thousands of people and forced half the pre-war Syrian population of 23 million from their homes.