AMSTERDAM - Assertions of chemical weapon use in Syria by Western and Israeli 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.

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," Michael Luhan, a spokesman for the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW) said.

With Syria blocking the UN mission, it is unlikely they will gain that type of access any time soon.

The White House and Western diplomats at the UN said they believe Syria had "probably" fired chemical munitions, but failed to name the chemical in question.

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