Barack Obama has declared the use of chemical weapons in Syria to be a "red line" or a "game changer" which could trigger military intervention by the US or Israel. But the latest claim by a senior Israeli intelligence officer is the most explicit and detailed statement yet made about their operational deployment.

In Damascus, Syrian government officials say chemical weapons were used by rebels in Khan al-Assal in Aleppo last month. Opposition sources say there is evidence of the use of some kind of gas by government forces fighting for parts of the capital under the control of insurgents.

Faisal Miqdad, Syria's vice foreign minister, told the Guardian last week that his government would only agree to a "real" UN investigation and not to a repeat of what had happened in Iraq under Saddam Hussein. "The game is clear," he said. "They want to cover up what happened in Aleppo and create fictitious issues." In a submission to UN member states the Syrian government warned earlier this month against "manoeuvres" by the UN secretariat, recalling "pre-fabricated allegations" against Iraq before the 2003 invasion. It appeared to be haggling over the terms of reference for any UN mission.

William Hague, Britain's foreign secretary, warned last week that it was urgent to investigate the issue after soil samples gathered from random sites in Syria were reportedly smuggled to the UK for testing. The evidence is said to suggest "some use of chemical weapons" but there is no certainty about who used them. According to a British official "the nightmare scenario" would be discovering that elements of Jabhat al-Nusra, a jihadi-type fighting group that has sworn allegiance to al-Qaida, had obtained and deployed a crude chemical weapons device – a provocation which the Israelis would find hard to ignore. "We are very, very close to the red line now," the UK official added.

Syria is believed to have stockpiles of mustard gas and the highly toxic nerve agent sarin – whose use was specifically mentioned by the Israeli intelligence officer. It is one of only a handful of countries which have failed to sign the chemical weapons convention, which internationally bans their use. A report citing Turkish, Arab and western intelligence agencies put Syria's stockpile at approximately 1,000 tonnes of chemical weapons, stored in 50 towns and cities. Israel also fears that chemical weapons might be transferred to Hezbollah, Bashar al-Assad's Lebanese Shia militia ally.

Speaking in Jerusalem after talks with Binyamin Netanyahu on 20 March, Obama said that the use of chemical weapons or their transfer to terrorist groups would be a "game changer". He added: "When you start seeing weapons that can cause potential devastation, and mass casualties, and you let that genie out of the bottle, then you are looking potentially at even more horrific scenes than we have already seen in Syria, and the international community has to act on that additional information."

But Syrian opposition sources have begun to scorn Obama for not standing by that commitment: "The US said that the use of chemical weapons was a red line for the Assad regime but the regime is using them and nothing has happened," protested Hisham Marwa, a senior member of the National Opposition Coalition.