Assertions of chemical weapon use in Syria by Western officials citing photos, sporadic shelling and traces of toxins do not meet the standard of proof needed for a UN team of experts waiting to gather their own field evidence.
Weapons inspectors will only determine whether banned chemical agents were used in the two-year-old conflict if they are able to access sites and take soil, blood, urine or tissue samples and examine them in certified laboratories, according to the Organisation for the Prohibition of Chemical Weapons (OPCW), which works with the United Nations on inspections.
That type of evidence, needed to show definitively if banned chemicals were found, has not been presented by governments and intelligence agencies accusing Syria of using chemical weapons against insurgents.
"This is the only basis on which the OPCW would provide a formal assessment of whether chemical weapons have been used," said Michael Luhan, a spokesman for the Hague-based OPCW.
With Syria blocking the UN mission, it is unlikely they will gain that type of access any time soon.
The head of the UN inspection mission, Swedish scientist Ake Sellstrom, will meet U.N. Secretary-General Ban Ki-moon in New York on Monday.
The United Nations wrote to the Syrian government again on Thursday to push for unconditional and unfettered access for the UN investigators, Ban's spokesman Martin Nesirky told reporters on Friday.
"The Secretary-General urges the Syrian government to respond swiftly and favorably so that this mission can carry out its work in Syria," Nesirky said. "You need to be able to go into Syria to be able to do that investigation properly."
"In the meantime the members of that team have been collating and analyzing the evidence and information that is available to date from outside," he said, adding that there was a concern "about the degradation of evidence" within Syria.
The White House on Thursday said the U.S. intelligence community has assessed with varying degrees of confidence that the chemical agent sarin was used by forces allied with President Bashar al-Assad. But it noted that "the chain of custody is not clear."
Questions around 'pysiological' samples
Ralf Trapp, an independent consultant on chemical and biological weapons control, said, "There is a limit to what you can extract from photograph evidence alone. What you really need is to get information from on the ground, to gather physical evidence and to talk to witnesses as well as medical staff who treated victims."
Sarin is a fast-acting nerve agent that was originally developed in 1938 in Germany as a pesticide. It is a clear, colorless, tasteless and odorless liquid that can evaporate quickly into a gas and spread into the environment, according to the US Centers for Disease Control and Prevention.
Because it evaporates so quickly, sarin presents an immediate but short-lived threat.
Sean Kaufman of the Center for Public Health Preparedness and Research at Emory University, a former biodefense expert for the CDC, said people who have been exposed to sarin most typically die or recover fully. Testing for sarin, he said, requires access to the environment where the nerve agent was used or the clothing of someone who was exposed.
The White House, which has called the use of chemicals weapons in Syria a "red line" for possible military intervention, said its assessment was partly based on "physiological" samples. But a White House official speaking on condition of anonymity declined to detail the evidence. It is unclear who supplied it.
Even if samples were made available to the OPCW by those making the assertions, the organisation could not use them.
"The OPCW would never get involved in testing samples that our own inspectors don't gather in the field because we need to maintain chain of custody of samples from the field to the lab to ensure their integrity," said Luhan.
Established to enforce the 1997 Chemical Weapons Convention, which bans the use of toxic agents in warfare, the OPCW has exhaustive rules on how inspectors collect and handle evidence, starting with the sealing of a site like a crime scene.
Multiple samples must be taken and there need to be "blank" samples from unexposed matter and tissue, to set a baseline against which levels of contamination could be determined.
The samples would be split, sealed and flown in dark, cooled air transports to up to three certified laboratories, including one at the OPCW's headquarters in The Hague.
A team of 15 experts, put together in response to a request from the U.N. Secretary General to investigate the claims, has been on standby in Cyprus for nearly three weeks.
Headed by Sellstrom, it includes analytical chemists and World Health Organisation experts on the medical effects of exposure to toxins.