Video report by ITV News Correspondent Paul Davies
The Syrian government and so-called Islamic State (IS) militants used chemical weapons during attacks in Syria in 2014 and 2015, a UN report has found.
The report - carried out in conjunction with the Organisation for the Prohibition of Chemical Weapons (OPCW) - said the Assad regime used chlorine gas in at least two attacks including one which killed children.
It also accused IS fighters of using mustard gas in an assault.
But weapons inspector Hamish de Bretton-Gordon told ITV News he suspected Assad had dropped chlorine on innocent civilians "at least 100 times".
He said: "I've seen reports of at least 100 chemical weapon attacks by Assad in the past three years.
"He uses chemical weapons when he's absolutely backed up against a wall and his regime is falling. It's his last ditch resort."
The UN-OPCW investigative team was formed by the UN Security Council a year ago to identify parties responsible for chemical attacks in Syria.
It examined nine cases in seven towns where chemical weapons were believed to have been used, and identified responsible parties in three cases.
The team said evidence in three other cases suggested government responsibility, but was not conclusive. Findings in three further cases were inconclusive.
Between December 2015 and August 2016, the UN-OPCW investigators said they received more than 130 new allegations from UN member states of use of chemical weapons or toxic chemicals as weapons in Syria.
US ambassador to the UN Samantha Power said the Syrian government had breached a September 2013 agreement under which the Assad regime agreed to relinquish its chemical weapons stockpile.
Power called on the Security Council to take "strong and swift action" against those accused of using chemical weapons.
France's ambassador to the UN, Alexis Lamek, also called for the Security Council to take action.
"When it comes to proliferation, use of chemical weapons, such weapons of mass destruction, we cannot afford being weak and the council will have to act," he said.
UK Foreign Secretary Boris Johnson said the report confirmed long-held beliefs by the British government that the Assad regime has been responsible for using chemical weapons in at least two attacks in Syria.
"There has been a clear pattern across many reports showing the use of helicopters and barrel bombs as delivery methods, attacking opposition-held areas with civilians - the overwhelming majority of victims," he said.
"Chemical weapons inflict excruciating pain and suffering. The Assad regime's indiscriminate and sustained use of them against their own people, including children, is horrific and must stop. Those responsible must be held to account."
The US National Security Council spokesman Ned Price added: "We continue to remove leaders from the battlefield with knowledge of these weapons and will target any related materials and attempts to manufacture such chemicals going forward."
The Security Council is scheduled to discuss the report on August 30, but it is not clear if any action will be agreed.
Although Moscow supported the establishment of the UN-OPCW investigation, it has blocked sanctions and other action against the Assad regime, which is a close ally.