AMSTERDAM (Reuters) - Inspectors have concluded that a “toxic chemical” containing chlorine was used in an attack last April in the Syrian town of Douma, at the time held by rebels but besieged by pro-government forces, the global chemical weapons agency said on Friday.

FILE PHOTO: The building of the Organization for the Prohibition of Chemical Weapons (OPCW) is pictured in The Hague, Netherlands, October 4, 2018. REUTERS/Piroschka van de Wouw/File Photo

The attack on April 7, 2018, killed dozens of civilians and prompted air strikes against the Syrian government by Britain, France and the United States.

Washington blamed the Syrian government and said it had used chemical weapons. Damascus denies having ever used chemical weapons.

During an investigation in mid-April, inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) visited two sites in Douma to interview witnesses and take samples, which have been analyzed in OPCW-affiliated national laboratories.

The investigation did not assign blame, but the information gathered provided “reasonable grounds that the use of a toxic chemical as a weapon has taken place on 7 April 2018”.

“This toxic chemical contained reactive chlorine. The toxic chemical was likely molecular chlorine,” the OPCW said in a statement.

Weaponizing chlorine is prohibited under the Chemical Weapons Convention, ratified by Syria in 2013, and is prohibited under customary international humanitarian law.

The OPCW has documented systematic use of the banned nerve agent sarin and chlorine in Syria’s civil war, now nearing its eighth year.

From 2015 to 2017 a joint U.N.-OPCW team had been appointed to assign blame for chemical attacks in Syria. It found that Syrian government troops had used the nerve agent sarin and chorine barrel bombs on several occasions, while Islamic State militants were found to have used sulfur mustard.

In June, the OPCW’s member states granted the organization new powers to assign blame for chemical weapons attacks, but that was not the mandate of the team that carried out the Douma inquiry.

The latest OPCW report “adds one more case to the scores of illegal chemical weapons attacks confirmed since 2013,” said Lou Charbonneau of the New York-based Human Rights Watch.

“It’s clear that the organization’s new unit for attributing blame for chemical weapons attacks in Syria has its work cut out. Those responsible for the use of these banned weapons should be unmasked and held to account.”

The OPCW is also looking into an alleged gas attack last November in Aleppo that reportedly made up to 100 people ill. The Syrian government and its ally, Russia, blamed that attack on insurgents.