UN chemical weapons experts in Syria
UN chemical weapons experts in SyriaINN:Reuters

In a report released on Friday, a UN team declared the Syrian government used chemical weapons in an attack for the third time in March 2015. In August, the team declared Syrian President Bashar Assad's government had used both mustard gas and chlorine gas, in three separate attacks, and had possibly been involved in an additional three chemical attacks.

"This independent investigation has concluded that the Assad regime has used chemical weapons against the Syrian people on at least three occasions, and that Daesh (ISIS -ed.) has used such weapons at least once," U.K. Foreign Secretary Boris Johnson said in a statement to NBC News.

"The attacks in Qamenas, Sarmin and Talamenes demonstrate the cruel barbarity of the Assad regime in this conflict. The use of chemical weapons by Daesh in Marea shows their complete disregard for human rights, and international norms and values," he added.

According to the report, the attack on Qamenas "was caused by a Syrian Arab Armed Forces helicopter dropping a device from a high altitude which hit the ground and released the toxic substance that affected the population."

Based on the smell and symptoms, investigators believe the substance may have been chlorine gas.

However, not enough evidence was available to positively determine the government was responsible for the other two attacks, carried out in 2014 and 2015.

"It is crucial to hold those who use or intend to use chemicals as weapons accountable for their acts, as it is fundamental to deter all those who continue to believe that there is something to be gained in the use of toxic chemicals as weapons," the panel said.

The United States, Britain and France want the U.N. Security Council to impose sanctions on the Assad regime for using chemical weapons. Russia refused to agree to such sanctions, however, claiming the evidence is not conclusive enough.

"The regime and its backers, notably Russia, are inflicting the worst bombardment seen yet on Aleppo, and other towns and areas in Syria, causing severe humanitarian suffering," said Johnson. "We are seeing heartbreaking scenes."

The Security Council is expected to discuss the report on Thursday.

ISIS terrorists have also used mustard gas as a weapon.

Syria was supposed to have destroyed its cache of chemical weapons, and had promised not to use them against its citizens. The country joined the Chemical Weapons Convention in 2013.