Employees of the advocacy group Human Rights Watch expressed skepticism Wednesday over a report that a State Department cable had concluded that the Syrian government used chemical weapons last month against rebel-held neighborhoods in the city of Homs.

The group, which has been at the forefront of documenting human rights violations perpetrated during Syria’s civil war, said it had received reports of suspected chemical-weapons use in Homs but had been unable to confirm that the incident had taken place.

“I shared these with our arms experts at HRW at the time, but based on the information available to us we have not been able to confirm that the government did in fact use chemical weapons,” Lama Fakih, a researcher for Human Rights Watch based in Lebanon, said in an email.

In Washington, State Department spokeswoman Victoria Nuland also rejected the report, saying U.S. authorities had investigated a tip that arrived in late December about a possible chemical-weapons attack in Homs but found “no credible evidence to corroborate or confirm chemical weapons.”

The issue of chemical weapons in Syria’s civil war is a sensitive one. President Barack Obama has said that the use of such weapons by the government of President Bashar Assad would trigger a so-far undefined American military response, and U.S. officials have used diplomatic and other channels repeatedly to warn the Syrian government not to use its chemical weapons.

The issue was raised again Tuesday, however, when a blog on the website of the journal Foreign Policy reported that a classified diplomatic cable from the U.S. consulate in Istanbul had concluded that chemical weapons had been used.

The report didn’t quote the cable itself. But it said an Obama administration official who’d read the cable said an investigation by a company hired by the United States had determined that Syrian doctors in Homs had made a “strong case” that the Syrian military had deployed a chemical agent. But the story also quoted the unnamed administration official as saying that U.S. officials weren’t “100 percent certain” that chemical weapons had been deployed.

Rami Abdurrahman, the head of the Syrian Observatory for Human Rights, a British-based group that tracks the fighting and casualties on both sides of Syria’s war, said he was familiar with the incident the cable referred to and that he too didn’t believe that chemical weapons had been involved. He noted that rebels frequently report the use of chemical weapons but that to date no such report has been confirmed.

“It’s not chemical weapons, but they used some kind of gas,” Abdurrahman said, noting that some of the people who’d been exposed to the gas had made full recoveries.

“If you have evidence I will report it, but I will not participate in propaganda,” Abdurrahman said.

Outside groups have documented the Syrian government’s use of other kinds of weapons that have been confused with chemical weapons, including incendiary cluster bombs, which cause fires that are difficult to extinguish and can cause severe burns, as well as skin and lung damage and death from inhaling the smoke they produce.

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