GENEVA (Reuters) - U.N. investigators said on Wednesday that they had documented three further uses of banned chlorine weapons by Syrian government forces that constituted war crimes, and urged major powers to help avert a “massacre” in the final battle for Idlib.

FILE PHOTO - Labels of the Organisation for the Prohibition of Chemical Weapons (OPCW) are seen iside a damaged house in Douma in Damascus, Syria April 23, 2018. REUTERS/ Ali Hashisho

The attacks caused injuries in the Damascus suburb of Douma and in Idlib in the northwest in January and early February, they said in their latest report.

They said they were still investigating a suspected chemical attack in Douma on April 7 that killed at least 49 people and wounded up to 650.

The United Nations has warned of a potential humanitarian catastrophe in Idlib, the last major rebel stronghold, if an expected offensive is carried out by the Syrian government and its allied Russian forces.

“The Commission of Inquiry demands that all parties to the conflict and those states who support them do everything in their power to prevent a massacre in Idlib,” the war crimes investigators said in a statement read out by panel chairman Paulo Pinheiro to a news conference.

They had no information that chemical agents might be used against the 2.9 million civilians living in Idlib, half of them refugees from elsewhere in Syria. The pocket is controlled by tens of thousands of rebel fighters, including around 10,000 Islamists designated as terrorists by the world body.

However, the three new cases bring to 33 the number of chemical attacks that the independent panel has documented since 2013 and attributed to the government, a U.N. official said. The use of chlorine in Douma on April 7, for which Western powers have blamed Syria, came as the government and its allies were in a final push to drive rebels out of the eastern Ghouta region.

“MUCH STRONGER RESPONSE”

John Bolton, U.S. President Donald Trump’s national security adviser, said on Monday that the United States, Britain and France had agreed that another use of chemical weapons by the Syrian government would result in a “much stronger response” than previous such incidents.

The April attack resulted in U.S., British and French missile strikes on sites believed to be linked to Syria’s chemical weapons program.

Pinheiro said the latest reports of air strikes on rebel-held areas of Idlib followed a previous pattern of “targeting civilian infrastructure and medical facilities”:

“For months we have been saying this concentration of population and armed groups transported or transferred to Idlib was a (time) bomb.”

Chlorine was dropped by government helicopters in Saraqeb in Idlib on Feb. 4, injuring at least 11 men, the report said, while women and children were injured in the Jan. 22 and Feb. 1 attacks on Douma.

“The Commission concludes that, on these two occasions, government forces and/or affiliated militias committed the war crimes of using prohibited weapons and launching indiscriminate attacks in civilian-populated areas in eastern Ghouta,” it said.

The use of chlorine as a weapon is prohibited under the Chemical Weapons Convention.

In addition to the 33 uses of chlorine weapons attributed to the government, the perpetrators of six others have not been sufficiently identified.

On the April 7 attack, the panel said the evidence so far pointed to a gas cylinder dropped by helicopter having struck a residential apartment building.