The panel was able to identify the perpetrators of three chemical attacks carried out in 2014 and 2015, but was unable to draw conclusions in the other six cases that it has been investigating over the past year.
The report from the Joint Investigative Mechanism (JIM) found that the Syrian regime dropped chemical weapons on two villages in northwestern Idlib province: Talmenes on April 21, 2014 and Sarmin on March 16, 2015.
In both instances, Syrian air force helicopters dropped "a device" on houses that was followed by the "release of a toxic substance," which in the case of Sarmin matched "the characteristics of chlorine."
The panel found that the Islamic State "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.
The Assad regime has repeatedly denied that it has used chemical weapons in Syria, but the report said that in all three cases, it had "sufficient information to reach a conclusion on the actors involved."
The JIM was set up by the Security Council a year ago to investigate the use of chemical weapons and for the first time to determine who is responsible for the attacks.
Most of the nine cases investigated pointed to the alleged use of chlorine gas in barrel bombs dropped from helicopters.
Britain, France and the United States had long maintained that only the regime has helicopters, but Russia, Damascus's ally, insisted that there was no concrete proof that Assad's forces carried out the attacks.
- US calls for swift action -
US Ambassador Samantha Power called for "strong and swift action" by the Security Council to follow up on the findings of the report.
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"It is essential that the members of the Security Council come together to ensure consequences for those who have used chemical weapons in Syria," she said in a statement.
In Washington, 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 work with our international partners to seek accountability through appropriate diplomatic mechanisms, including through the United Nations Security Council," he added.
The report "states clearly that the Syrian regime and Daesh have perpetrated chemical attacks in Syria," French Deputy Ambassador Alexis Lamek told reporters.
"When it comes to proliferation, the use of chemical weapons, of such weapons of mass destruction, we cannot afford to be weak. The council will have to act."
The Security Council is due to discuss the report on Tuesday and could decide to impose sanctions on Syria or ask the International Criminal Court to take up the matter as a war crime.
But many diplomats say Russia would be unlikely to back such a move, despite the JIM's strong findings of chemical weapons use in the three cases.
The panel recommended further investigation of three other cases of suspected chemical weapons on the village of Zafr Zita, in Hama province, on April 28, 2014, and on two towns in Idlib: Qmenas on March 16, 2015 and Binnish on March 24, 2015.
The 24-member team said there was insufficient information to reach a conclusion in three other cases and recommended that there be no further investigation of those suspected attacks.
Syria agreed to get rid of its chemical stockpile and to refrain from making any use of toxic substances in warfare when it joined the Chemical Weapons Convention in 2013, under pressure from Russia.
The findings prompted immediate calls for the perpetrators to face justice.
"The UN Security Council should now ensure that those responsible for these attacks are brought to justice in a court of law," said Louis Charbonneau, Human Rights Watch's UN director.