An international inquiry has identified two Syrian Air Force helicopter squadrons and two other military units it holds responsible for chlorine gas attacks on civilians, according to Reuters.
The finding by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW), the global chemical weapons watchdog, is based on Western and regional intelligence, the news agency said quoting a diplomatic source.
"It was the 22nd Division, the 63rd Brigade and the 255 and 253 squadrons of the Syrian government," the envoy said.
The identification of specific military institutions responsible for attacks could strengthen a push by some Western members of the U.N. Security Council for a robust response, focused on sanctions and accountability.
Syrian regime has denied using toxic gas on the battlefield.
The year-long inquiry, which is investigating reports of attacks between April 11 2014 and August 21 2015, is due to submit its fourth report to the UN Security Council next week.
The third report, in August, blamed Syrian regime forces for two chlorine gas attacks and DAESH terrorists for using sulfur mustard gas.
It is unclear whether the fourth report will assign blame to individuals. The inquiry has focused on nine attacks in seven areas of Syria.
Eight of the attacks investigated involved the suspected use of chlorine.
"There is no indication that any opposition groups used chlorine," the dilpomat said.
Syria agreed to destroy 1,300 tonnes of declared chemical weapons in 2013 under a deal brokered by Moscow, Damascus's main international backer, and Washington, which supports the Syrian opposition.
The latest suspected use of chlorine gas was last week, when rescue workers and a monitoring group said there were dozens of cases of suffocation in an opposition neighborhood in the city of Aleppo.
Chlorine's use as a weapon is prohibited under the 1997 Chemical Weapons Convention, which Syria joined in 2013.
If inhaled, chlorine gas turns to hydrochloric acid in the lungs and can kill by burning the lungs and drowning victims in the resulting body fluids.
The new finding, blaming specific military units, could set the stage for a showdown at the Security Council pitting the United States, Britain and France against Russia and China.
Some Western diplomats worry that the Security Council could respond weakly to the reported chemical weapons attacks or that the issue could be sidelined because of the fragility of a Syria ceasefire deal agreed by Moscow and Washington.
Some diplomats said U.N. and OPCW investigators may ask for more time to finish their fourth report, in which case the Security Council may renew the mandate for the inquiry for a short time.
Samantha Power, US ambassador to the United Nations, said on Thursday it was important those compiling the inquiry "go as far as they can" to identify individuals and entities involved in the attacks. The United States planned "to push to extract from the council as much as we can" on a response, she said.
French Foreign Minister Jean-Marc Ayrault said on Tuesday he would fight vigorously for sanctions on those responsible for the gas attacks.