Boris Johnson has called for those responsible for using chemical weapons in Syria to be held to account after a UN team concluded the Assad regime and Islamic State had used the banned munitions.
A report found that the Syrian government and the terror group had carried out chemical attacks in the country in 2014 and 2015.
It suggests the Syrian government used chlorine gas in two attacks while Islamic State is believed to have used mustard gas in one attack.
The Foreign Secretary said: "This UN report confirms for the first time what we have long believed: the Assad regime has been responsible for the use of 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.
"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 report also identifies Daesh as a perpetrator. The Global Coalition is making progress in defeating Daesh and the UK will continue to play a leading role."
Mr Johnson said the UK will work with its international partners to secure a UN resolution to condemn those responsible for the use of chemical weapons in Syria.
The UK will also work to prevent future use of the weapons, Mr Johnson said.