Washington: The White House condemned the Assad regime's "defiance" of international norms after United Nations experts found the Syrian army had conducted three chemical weapons attacks against its own people in 2014 and 2015.
Of the nine alleged chemical attacks it is considering in its ongoing probe, the UN-led joint investigative mechanism has now attributed three to the Syrian government and one to the Islamic State group.
"We condemn in the strongest possible terms the Assad regime's defiance of the longstanding global norm against chemical weapons use and Syria's abrogation of its responsibilities under the Chemical Weapons Convention, which it joined in 2013," National Security Council spokesman Ned Price said in a statement.
"The Syrian regime has violated the Chemical Weapons Convention and UN Security Council Resolution 2118 by using industrial chlorine as a weapon against its own people."
He warned the United States was continuing to work with its allies to hold those responsible to account.
"We strongly urge all UN member states and parties to the Chemical Weapons Convention, including Russia and Iran, which continue to conduct military operations on behalf of the Assad regime, to unequivocally support these efforts and sustain our shared commitment to the international standard against chemical weapons use."
Washington and Moscow are in the midst of a major diplomatic spat over Russian support for the Assad regime, with relations reaching a new low not seen since the Cold War.
"Russian military and economic support to Syria enables the Assad regime to continue its military campaign against its own people, which it has done in violation of international law as demonstrated by the findings of this report," Price said.
"Together, we must send a strong signal that the use of weapons of mass destruction will not be tolerated by the international community."