Foreign powers that sympathize with Syria’s rebels accused the regime of President Bashar Assad this week of deploying chemical weapons in the country’s two year civil war.

Israel’s top military intelligence analyst said on Tuesday that forces loyal to Assad had used chemical weapons, probably the nerve gas sarin, against rebel fighters.

Qatar’s prime minister Hamad bin Jassim bin Jaber Al Thani, who also serves as the emirate’s foreign minister, similarly told reporters in Washington a day after speaking with President Barack Obama there, “He used chemicals and there is evidence.”

Britain’s Foreign Office corroborated the claim the next day when it claimed to have “limited but persuasive information from various sources showing chemical weapon use in Syria, including sarin.”

Later in the day, American officials said they believed with “varying degrees of confidence” that sarin had been used by Assad’s army. However, they noted that “the chain of custody is not clear.”

France and the United Kingdom asked the United Nations last month to investigate such allegations of chemical weapons use.

The Syrian Network for Human Rights’ Sami Ibrahim backed the allegations, telling USA Today that Assad’s forces had used chemical weapons on “ten separate locations in Syria” since December.

We have videos of those killed, we have photos, we have testimony from the eyewitnesses, from the doctors inside the hospitals.

Ibrahim urged Western powers to expand their support for the opposition which has so far been confined to “nonlethal” aid. “We heard hundreds of statements against Bashar al-Assad, that he’s a criminal and he should leave. But give me the real action against him!”

Britain and the United States have repeatedly warned Assad that the use of chemical weapons would cross a “red line” and could trigger Western intervention, although leaders haven’t made clear why.

The United States especially have been reluctant to arm rebel fighters for fear of propping up a jihadist insurgency. The New York Times reported in October that most of the communications equipment and weapons supplied by the United States and its allies in the region, including Qatar, Saudi Arabia and Turkey, ended up in the hands of religious extremists.

Dina Esfandiary of the International Institute for Strategic Studies is skeptical of the claims, telling Foreign Policy that sarin is not particularly suited to the sort of urban warfare that has wrecked Syria’s cities. “Sarin in particular is very volatile (evaporates easily), which means it presents an immediate but short-lived threat,” she told the magazine.

Syria seems to have the capacity to deliver nerve gas with its rockets and missiles. But Syria’s missiles in particular are inaccurate and have small payloads. The speed at which missiles hit targets make it difficult to use them to disperse chemical weapons homogeneously.

Assad was previously accused of deploying chemical weapons in March. No definitive proof emerged at the time.