What is being alleged and by whom?
The US, the UK, France and Israel all say there is evidence pointing to the use of the nerve agent sarin in Syria. The US secretary of state, John Kerry, said the US was looking into two cases, but did not specify where. British officials talk about three incidents in particular, at Khan al-Assal near Aleppo, in Homs and near Damascus. All the allegations involve relatively small-scale attacks, with the numbers of alleged victims in the dozens at most rather than hundreds.
Rebel groups have made several claims over the course of the two-year conflict that the regime has used chemicals against its fighters and civilians. On 19 March both the Assad regime and the rebels claimed nerve gas had been used against their forces at Khan al-Assal. British officials say Syrian army troops appear to have been affected in that incident but suggest it was either a case of "friendly fire", a projectile going astray or a deliberate attempt to implicate the rebels.
What is sarin?
It is a manmade chemical compound similar to organophosphate pesticides but much more toxic. It was first developed as a pesticide in Germany in 1938. It is a colourless, odourless liquid that easily evaporates as a vapour.
Exposure leads to muscle spasms, heavy sweating, and ultimately respiratory failure and death. It is believed to have been used in the Iran-Iraq war in the 80s, possibly in the attack by Saddam Hussein's regime against the Iraqi Kurds in Halabja in 1988, and in attacks on the Tokyo metro in March 1995 by the Aum Shinryko cult, in which 13 people died.
How strong is the evidence that it has been used in Syria?
There appear to be two types of evidence: physical samples, of soil from the scene of the alleged attacks and possibly human tissue samples too; and videos, photographs and witness accounts.
The letter issued by the White House to two senators on Thursday makes it clear the evidence is far from conclusive. There are questions over the "chain of custody" of the physical evidence – ie the US analysts cannot guarantee the provenance of the samples they have been given because they were collected and handed over by someone else, either another government or an opposition faction.
According to a report by the McClatchy news agency in the US, the soil sample examined by American experts is minuscule and contains a byproduct of sarin that could also be from fertiliser production.
Some of the videos in circulation online show alleged victims foaming at the mouth, but that is not listed as a sarin symptom on the website of the US Centres for Disease Control and Prevention.
Richard Guthrie, a British chemical weapons expert and former head of the Chemical and Biological Warfare Project of the Stockholm International Peace Research Institute, said: "That [foaming at the mouth] would not be indicative of use of nerve agents but is more likely to be a sign of a choking agent such as phosgene being used, if anything were used. Phosgene is a widely used industrial chemical as well as being a first world war-era chemical weapon."
Jean-Pascal Zanders, an expert at the EU Institute for Strategic Studies, said: "It's not possible that what is being shown to the public is a chemical weapons attack. The video from Aleppo showing foaming at the mouth does not look like a nerve agent. I'm wholly unconvinced."
Will we ever know for sure whether this was a sarin or other chemical attack?
Possibly. The UN has launched an investigation in co-operation with the Organisation for the Prevention of Chemical Weapons (OPCW), the world's experts on the subject. However, because of disagreements with the Syrian government, some of the investigators are still in Cyprus waiting for a green light to travel, and some have returned to their home countries. The investigators are due to interview and possibly take samples from victims in refugee camps, and to examine samples taken by the UK and France, but it is unclear when that would happen.
OPCW protocols for judging whether a chemical weapon has been used are very strict. Its own investigators would have to have taken the specimen in tightly controlled circumstances, the chain of custody would have to be strictly monitored and the laboratories doing the analysis would have to OPCW-certified. It appears unlikely that the evidence now in the hands of western states would pass that threshold.
Guthrie said blood and tissue samples from victims might still provide solid evidence as it was harder to fake the chemical metabolites left in the body as a result of sarin exposure than to spike a soil sample with sarin itself.
Nobody expects a result from the UN inquiry any time soon.
If sarin or another chemical weapon is proved to have been used, will that cross the "red line" laid down by Barack Obama and trigger military action?
The White House letter makes it clear that the current evidence falls below the US administration's standard of proof. Even if proof is found, the scale of the use may be a factor. When Obama originally laid down his "red line" he said it could come into effect if "a whole bunch" of chemical weapons were moved or used.
The very limited nature of the recorded incidents, if proved to involve chemical weapons, may mean that Damascus is testing the boundaries of what is acceptable to Washington, or it may signify that chemical weapons have been used inadvertently by local commanders. In the Iran-Iraq war, Guthrie said, artillery commanders sometimes used chemical rounds by mistake because they were not clearly marked.
Furthermore, Obama is not saying explicitly that the US would take unilateral action. The first step is likely to be to go to the UN security council once more and to try to persuade Russia and China to back punitive action against the Syrian government. Pressure on the Obama administration to take some kind of action will mount if the evidence strengthens, but that may take a considerable length of time, if it ever happens at all.