An independent United Nations commission has said the Syrian government is likely to have used chlorine gas to attack civilians.
It added the Islamic State group committed crimes against humanity with attacks on civilians in two cities in the country's north and west.
The report from the commission, which has been tasked with investigating potential war crimes in the country, marks the first time the UN has assigned blame for the use of the chemical agent.
Specifically, the commission said government forces loyal to President Bashar Assad are likely to have unleashed chlorine on civilians in northern Syrian villages eight times in April.
The commission also noted widespread and systematic civilian killings by Islamic State, which now controls a swath of north and eastern Syria, in the northern city of Aleppo and in the western city of Raqqa where the group has its headquarters.
The findings mean UN officials now believe Islamic State has committed crimes against humanity in Syria and Iraq, the two countries in which the group has carved out a self-styled caliphate.
"This is a continuation - and a geographic expansion - of the widespread and systematic attack on the civilian population," according to the report of the four-member commission that is chaired by Brazilian diplomat and scholar Paulo Sergio Pinheiro.
The commission also said Assad's government forces continue to perpetrate crimes against humanity - the most serious and systematic type of widespread crime against civilians - through massacres and systematic murder, torture, rape and disappearances.
On Monday, UN High Commissioner for Human Rights Navi Pillay said the Islamic State fighters reportedly killed up to 670 prisoners in Mosul and committed other horrific abuses in Iraq that amount to crimes against humanity.
Ms Pillay said the Islamic State group and other fighters allied with it are daily committing "grave, horrific human rights violations" in Iraq such as targeted killings, abductions, trafficking, slavery and sexual abuse in an aggressive push to gain a firm grip on Iraq's northern and eastern provinces.
The latest report, based on 480 interviews and documentary material, cited dozens of documented public executions in Aleppo and Raqqa during the bloody and complex Syrian civil war that the UN says has killed more than 190,000 people.
Crowds of people including children have reportedly watched as the group's fighters pronounce mostly adult men guilty of violating religious laws, and then behead them or shoot them in the head at close range. The purpose, according to the commission, is "to instil terror among the population, ensuring submission to its authority".
Photos posted online show the aftermath of the Islamic State group's takeover of the Tabqa air base in Raqqa province. In one photo, masked gunmen can be seen shooting seven men kneeling on the ground in front of them.