Western powers have warned Damascus that using chemical weapons is a red line beyond which it could be necessary to intervene to protect civilians and prevent weapons from falling into terrorist hands.
But analysts say it can be difficult to prove chemical weapons are being used in conflicts, particularly if their use is on a small scale and international inspectors are barred from the country.
"Without allowing...inspectors in, that would definitely create a big hurdle to providing any kind of conclusive evidence that weapons had been used," says Gregory Koblentz, a chemical weapons expert at the Council on Foreign Relations in Washington.
"Really, the gold standard is to have inspectors, trained experts, on the ground who have unimpeded access to the site of the alleged incident, to interview victims and witnesses, to take soil samples, take blood samples, and have them tested in multiple independent certified laboratories."
Western powers want just that: for UN inspectors to go to Syria and investigate multiple places where chemical weapons are alleged to have been used.
Both Damascus and Syrian rebels accuse each other of using chemical weapons, and international concern over the issue has grown dramatically in recent days.
The United States said last week it has evidence that chemical weapons have been used inside Syria.
But U.S. President Barack Obama said firmer evidence is needed to know how the chemical agents were used, when they were used, and who used them.
To get such evidence, the UN has an inspection team waiting in Cyprus to go to Syria.
However, Damascus, which never signed the 1997 Chemical Weapons Convention banning the use of toxic agents in warfare, is not required to admit them.
Prospects for admitting the UN experts now appear to be dimming as Damascus and the UN quarrel over how much access to Syrian sites the team should have.
Damascus originally invited the UN to investigate the government's own charges that rebels used chemical weapons in fighting in the village of Khan al-Assal outside the northern city of Aleppo.
But when the UN asked for full access to the country, Damascus refused, saying they could only come to the one site.
Now, as the United States, Britain, France, and Israel speak of more evidence of chemical weapons use, Syrian officials are balking further.
Syrian Information Minister Omran al-Zoubi told Russian TV on April 26 that the Americans "want to manipulate the issue...to repeat the Iraq example."
Western concern over Iraqi weapons of mass destruction, which later proved unfounded, motivated a U.S.-led coalition to overthrow Saddam Hussein in 2003.
Alternative Evidence Gathering
It now seems likely that efforts to get weapons inspections into Syria will become still more deadlocked in the weeks ahead. And in the absence of ground inspections, evidence will have to be obtained by other, potentially less conclusive means.
The main alternative is for parties within Syria to smuggle evidence of chemical weapons use to inspectors outside the country.
Indeed such samples, which can take the form of blood and urine obtained from victims, are believed to be partly behind current Western concerns that chemical weapons are being used.
But as Koblentz notes, the evidence such samples provide can only be accepted as firm after a host of difficult questions about their handling has been answered.
"The problem with that approach is what is called the chain of custody," he says. "How do you know that the samples that wind up in the laboratory for analysis really came from the site where they are said to have come from, [that they] really came from these particular victims, from this particular time, and were stored properly and were not tampered with in transit to the laboratory doing the analysis?"
But there are other problems, too. Direct traces of chemical agents become harder to detect as time passes after their use.
An example of this is sarin -- a nerve agent widely suspected to have been used in Syria and which was originally developed as a pesticide.
Sarin kills by shutting down the nervous system of its victims, but it is a clear, colorless, tasteless, and odorless liquid that evaporates quickly into a gas and disperses into the environment.
The Need For Speed
According to Koblentz, this puts a premium on gathering and examining evidence quickly, something that is difficult under wartime conditions.
"You want to have the evidence collected and analyzed as soon as possible to make sure you are able to get the most confidence in your findings," he says. "But even when sarin degrades it will turn into other chemical compounds that can also provide signatures that sarin used to be present. So even if the presence of sarin cannot be detected, you might see degradation products in the soil or certain metabolites in the blood or in urine samples that would be indicative of exposure to sarin. So, that lengthens the window a bit but, generally speaking, you do want to have the sample analysis done as soon as possible.”
Koblentz notes that the window for detecting traces of sarin is days or weeks, not months.
For all these reasons, it is safe to say that the battle to establish conclusively whether chemical weapons are being used in Syria has only just begun.
It is a battle in which all sides -- from Syrian officials to Syrian rebels and from Western powers to Damascus’s ally Russia -- know that the stakes are huge because the outcome could determine whether outside forces finally intervene in Syria.
Moscow stressed that it would draw a red line of its own by insisting upon the highest levels of evidence.
Russian Foreign Minister Sergei Lavrov said on April 29 that "the issue of weapons of mass destruction is too serious and it shouldn't become an object of games. Any speculations at the expense of this issue pursuing geopolitical goals are unacceptable."