The Joint Investigative Mechanism (JIM) set up by the Security Council last year appealed for more information from world governments including from the Syrian regime to determine those responsible for the attacks.
The JIM is the most concrete UN effort to date to establish some accountability in the five-year war that has left more than 280,000 dead and displaced over 11 million people.
"Investigations into the nine cases under consideration by the mechanism have all progressed," said the JIM in a report to the Security Council.
But identifying those responsible "continues to be dependent on sufficiency of information", said the report obtained by AFP.
The 24-member team is due to present its findings in August, when its one-year mandate ends, but the council could ask the panel to continue its work if it does not produce a list of perpetrators by then.
The panel is looking into nine attacks using toxic chemicals that took place in 2014 and 2015 in the Hama, Idlib and Aleppo provinces.
- Chlorine gas attacks -
The report did not provide details, but most of the cases point to the alleged use of chlorine gas in barrel bombs, that the West blames on the Syrian regime.
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These took place in Kafr Zita in Hama province while other attacks were in villages in Idlib including Talmenes, Al-Tamana, Qmenas, Binnish and Sarmin.
A more recent attack in Marea on August 21, 2015, pointed to the likely use of mustard gas by Islamic State militants.
The Organization for the Prohibition of Chemical Weapons (OPCW) has concluded that chlorine was probably used as a weapon on opposition-held villages, killing 13 people.
The OPCW did not assign blame for the attacks, among the many horrors to have been documented Syria's war.
The panel led by Argentine expert Virginia Gamba has visited seven countries, three of them in the Middle East, as part of its investigation and is planning to visit three more in June.
Investigators traveled to Syria twice this year, in March and May, to question government officials and interview witnesses.
The panel is working with two forensics institutes to analyse material and has consulted a number of laboratories on chemical substances and munitions.
Even if the panel comes up with a list of names, the Security Council would have to adopt a new resolution to impose sanctions or take action against the perpetrators.
That appears unlikely after Russia and China in 2014 blocked moves to refer Syria to the International Criminal Court for war crimes.