The Syrian government and Islamic State used chemical weapons during attacks in Syria in 2014 and 2015, a UN report has concluded.
An international team blamed the government of Bashar al Assad for using chlorine gas in two attacks.
Islamic State used mustard gas during another attack, the Organization for the Prohibition of Chemical Weapons-United Nations Joint Investigative Mechanism (JIM) found.
Three other attacks appeared to have been carried out by the government but it could not be certain, the team said in its report.
JIM was given a mandate by the UN Security Council to identify those responsible for chemical weapons attacks in Syria.
The US ambassador to the United Nations Samantha Power called the use of chemical weapons "a barbaric tool, repugnant to the conscience of mankind".
She urged the Security Council to take "strong and swift action" against those who had carried out the attacks.
The experts' findings mirror "numerous other confirmed cases of chemical weapons use across Syria, and countless other allegations of such use, including as recently as several weeks ago," the ambassador added.
France's ambassador to the UN Alexis Lamek also called for the Security Council to take action.
He said: "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."
In a statement, Ned Price - spokesperson for the White House's National Security Council - said: "It is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people in violation of the Chemical Weapons Convention and UN Security Council Resolution 2118.
"We condemn in the strongest possible terms the Asad regime's use of chlorine against its own people," he added.
The Security Council is due to discuss the JIM report on 30 August but analysts say it is not clear whether any action is likely to be agreed.
Russia, although it supported the establishment of the JIM, is a close ally of Syria and has previously blocked sanctions and other council action against President Assad's government.
One of the Chlorine attacks took place in Talmenes, in the Idlib governorate, on 21 April, 2014 and the other in Sarmin, in the same province, on 16 March, 2015, JIM found.
The mustard gas attack occurred in Marea, Aleppo governorate, on 21 August, 2015, when Islamic State fighters were attacking rebels.
The report said that between December 2015 and August 2016 JIM received more than 130 new allegations that chemical weapons or toxic chemicals, including sarin, VX nerve gas and other, were used as weapons in Syria.
It follows an earlier report by UN weapons inspectors that found "clear and convincing evidence" that chemical weapons were used in Syria.
It had been hoped that President Bashar al Assad's regime had destroyed its stockpile of weapons after the government agreed to an international plan following a global outcry over deadly chemical attacks in a Damascus suburb.
