A United Nations (UN) report has accused Syria of using chemical weapons eight times in April - and highlights mass atrocities by Islamic State militants.
The report by the independent Commission of Inquiry said it believed chlorine, dispatched in barrel bombs, was the agent used on multiple occasions.
It also highlighted mock crucifixions, public executions, amputations and the beheading of children by Islamic State militants in parts of Syria.
"Mass atrocities by government forces and non-State armed groups continue to take place in Syria, causing immeasurable suffering to civilians", the report says.
The accusations came amid reports that Syrian rebels had seized control of the Syrian crossing with the Israeli-occupied Golan Heights.
The Syrian Observatory for Human Rights said there had been heavy fighting between rebels - including the al Qaeda affiliated al Nusra Front - and the army.
An Israeli army officer stationed in the Golan Heights has been injured after errant fire, sources said.
The UN's 45-page report accused the Syrian regime of dropping barrel bombs on residential areas and highlighted atrocities meted out by Islamic State members.
"Reasonable grounds exist to believe that chemical agents, likely chlorine, were used on (northern Syrian villages) Kafr Zeita, al Tamana and Tal Minnis in eight incidents within a 10-day period in April," it said.
"Witnesses saw helicopters drop barrel bombs and smelled a scent akin to domestic chlorine immediately following impact," it pointed out.
Victims suffered "symptoms compatible with exposure to chemical agents, namely vomiting, eye and skin irritation, choking and other respiratory problems".
It marks the first time the UN has assigned blame for the use of the chemical agent.
'Boys beheaded for smoking'
Bashar al-Assad and the opposition have accused each other of using chemical agents, including chlorine, in the bloody uprising, that began in March 2011.
The report also outlines atrocities being carried out by IS militants in Syria, including public executions, amputations, lashings and mock crucifixions.
The document describes beheadings of boys as young as 15 and men flogged for smoking or accompanying an "improperly dressed" female relative.
Women have been publicly lashed for not following the group's strict dress code, the report says.
IS is also recruiting and training children as young as 10, with teens being used in active combat and suicide-bombing missions, the report said.
"This is a continuation - and a geographic expansion - of the widespread and systematic attack on the civilian population," according to the report.
The four-member commission was created three years ago by the UN Human Rights Council to investigate abuses committed in the war.
Syria has said it is ready to cooperate with the international community in the fight against the militants who have taken over areas straddling both Iraq and Syria.
