Western intelligence agencies fear they can no longer prove for certain whether the Syrian government was responsible for alleged chemical weapon attacks, because initial samples and evidence trails have degraded over time.
Instead, Britain and the US are likely to have to wait for fresh evidence from further attacks before deciding whether to take a military response against the Assad government.
Philip Hammond, the British defence secretary, revealed the shortcomings in recent reliance on soil and blood samples ahead of talks with his US counterpart Chuck Hagel at the Pentagon on Thursday.
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Britain currently supplies non-lethal assistance such as armoured 4x4s, body armour and night-vision goggles, while countries such as Saudi Arabia are know to have supplied guns. But Hammond said the significance of a US decision to lift an arms embargo would be to provide international "leadership" for the rebels.
full: http://www.guardian.co.uk/world/2013/may/02/us-uk-syria-chemical-weapons-claims