The Syrian government used chlorine gas in two attacks and Isis militants used mustard gas in one attack, the team from the United Nations and the world body's chemical weapons watchdog said.
Samantha Power, US ambassador to the UN, called the use of chemical weapons "a barbaric tool, repugnant to the conscience of mankind," and urged the Security Council to take "strong and swift action" against the perpetrators.
She accused the Syrian government of violating a September 2013 resolution which orders the council "to impose measures" under Chapter 7 of the UN Charter for "any use of chemical weapons by anyone in the Syrian Arab Republic". Those measures usually mean sanctions, and Chapter 7 can be militarily enforced.
In September 2013, Syria accepted a Russian proposal to relinquish its chemical weapons stockpile and join the Chemical Weapons Convention. The move averted a US military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta the previous month.
A year ago, the UN Security Council established the Organisation for the Prohibition of Chemical Weapons-United Nations Joint Investigative Mechanism - JIM - to identify those responsible for chemical attacks in Syria.
Jim investigated nine cases in seven towns where an OPCW fact-finding mission found that chemical weapons had probably been used.
It determined responsibility in three cases, said three attacks pointed towards government responsibility but were not conclusive, and described three others as inconclusive.
US National Security Council spokesman Ned Price said: "It is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people."
The United States will seek accountability at the UN and the OPCW and has placed "a high priority" on targeting Isis' chemical weapons capabilities, he added.
The US captured an Isis leader involved in manufacturing such weapons in March and used information from him for air strikes to reduce its ability to use chemical weapons, he said.
"We continue to remove leaders from the battlefield with knowledge of these weapons and will target any related materials and attempts to manufacture such chemicals going forward," Mr Price said.
Ms Power said Jim's findings mirrored "numerous other confirmed cases of chemical weapons use across Syria, and countless other allegations of such use, including as recently as several weeks ago".
In the report, Jim said that between December 2015 and August 2016 it received more than 130 new allegations from UN member states of the use of chemical weapons or toxic chemicals as weapons in Syria.
It said 13 alleged the use of sarin, 12 mustard gas, four VX nerve gas, 41 chlorine, and 61 other toxic chemicals.
"The information suggests the involvement of both the government of the Syrian Arab Republic and other actions in these alleged incidents," the report said.
Russia, a close ally of Syria, has blocked sanctions and other council action against President Bashar al-Assad's government, but Moscow supported the establishment of Jim.
Louis Charbonneau, the UN's director at Human Rights Watch, said the council must ensure that those responsible for "the sickening, illegal use of chemical weapons in Syria ... are brought to justice in a court of law".
"Without accountability, the cycle of abuses in Syria by all parties to the conflict - whether by chemical or conventional weapons - will continue unabated," he said.
According to the report, obtained by The Associated Press, Jim found the Syrian government responsible for two chlorine attacks in Idlib governorate, one in Talmenes on April 21, 2014 and one in Sarmin on March 16 2015.
It also said Isis was "the only entity with the ability, capability, motive and the means to use sulphur mustard" gas in Marea in Aleppo governorate near the Turkish border on 21 August, 2015. At the time, Isis fighters were attacking rebels.
Jim said three attacks indicated possible government involvement - in Kfar Zita in Hama governorate on 18 April, 2014, and in Qmenas on 16 March, 2015 and Binnish on 24 March, 2015, both in Idlib governorate. It said all three needed further investigation.
Ms Power said the United States expected Jim to continue investigating the remaining confirmed cases and any others referred by the OPCW fact-finding mission.
The Security Council will discuss the Jim report on 30 August, but whether it will take any action remains to be seen.
Additional reporting by AP and PA
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