A UN investigative panel is pressing the Syrian government to hand over the names of commanders, military units and other entities suspected of carrying out chemical attacks, according to a report released Friday.
The joint panel of the United Nations and Organization for the Prohibition of Chemical Weapons (OPCW) has concluded that Syrian forces were responsible for three chlorine bombings of villages in 2014 and 2015.
In its latest report to the Security Council, the panel said it had formally asked Syria to provide details of air operations, specifically at two air bases from which helicopters carrying the chlorine bombs lifted off.
Investigators are seeking the "names of specific Syrian Arab Armed Forces units and any entity outside the Armed Forces" listed on flight plans, said the report by the UN-OPCW Joint Investigative Mechanism (JIM).
"Such information is of great importance, given that commanders are responsible for any assets under their control," said the report. The Syrian government has ignored the request, it added.
Syria has denied using chemical weapons in the nearly six-year war while its ally Russia, a permanent Security Council member, has dismissed the panel's findings as "inconclusive".
France and Britain are pushing the Security Council to impose sanctions on those responsible for chemical attacks and to ban sales of helicopters to Syria.
The panel said it was awaiting further determinations from the OPCW, which carries out fact-finding missions to establish whether chemical attacks took place, but does not attribute blame.
In a report in December, the OPCW said it had investigated allegations of a chemical attack in Aleppo on August 2 last year, but was unable to "confidently determine" whether a chemical was used as a weapon.
UN member-states have come forward with nine allegations of chemical weapons use or possession, including one alleging a chlorine attack carried out by an armed opposition group in Aleppo in December, the report said.
Three of those allegations involve the Islamic State (IS) militant group. The information has been sent to the OPCW, which may decide to investigate further.
In a previous report, the panel concluded that IS had used mustard gas in an attack in 2015.
Human Rights Watch this week said it had collected witness accounts, photographs and video of at least eight chemical attacks carried out by Syrian forces in the final weeks of the battle for Aleppo late last year.