The White House and its NATO allies vowed to hold Syria and the Islamic State accountable for chemical weapons attacks documented by a United Nations investigation.
"It is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people," National Security Council spokesman Ned Price said following press reports revealing details of the UN's investigative report on August 24.
"The United States will work with our international partners to seek accountability through appropriate diplomatic mechanisms, including through the United Nations Security Council," he said.
"We urge all UN member states and parties to the Chemical Weapons Convention, including Russia and Iran, to participate in this effort."
The UN investigation established that President Bashar al-Assad's forces carried out at least two chemical attacks in Syria and that IS militants used mustard gas as a weapon.
The investigation determined that Syrian government helicopters dropped chlorine on the towns of Talmenes on April 21, 2014, and Sarmin on March 16, 2015.
The investigations also determined that the IS "was the only entity with the ability, capability, motive, and the means to use sulphur mustard" in an attack on the town of Marea in northern Aleppo province on August 21, 2015.
While UN investigators were able to identify the perpetrators of three chemical attacks carried out in 2014 and 2015, they were unable to draw conclusions in another six cases. They said they will continue investigating those cases.
Price said the findings "present yet another opportunity for all nations to speak with one voice to address these heinous crimes and to make it clear that the use of chemical weapons is intolerable."
His statement sent a veiled message to Russia, which worked with the United States in 2013 to try to eliminate Syria's stockpile of chemical weapons and has backed Syria in its five-year war against rebel forces.
President Barack Obama recently warned Russia it faces international rebuke if it continues to make common cause with Syria's brutal regime.
Russia and Syria did not immediately respond to the UN report.
Russia and China in the past have fended off moves by the United States, Britain, and France to impose UN Security Council sanctions on Syria.
The 15-member Security Council is due to discuss the report on August 30.
"It is essential that the members of the Security Council come together to ensure consequences for those who have used chemical weapons in Syria," U.S. Ambassador to the UN Samantha Power."We strongly urge all states to support strong and swift action."
France and Britain also said they will push for UN action.
"When it comes to proliferation, use of chemical weapons, such weapons of mass destruction, we cannot afford being weak and the council will have to act," said France's UN Ambassador Alexis Lamek.
"The use of these weapons is abhorrent and we unequivocally condemn those who unleash them," British UN Ambassador Matthew Rycroft said. "This council must stand ready to demonstrate a robust response to this report."