US ponders Syria options but urges caution over chemical weapons claims White House spokesman Jay Carney said: 'We need to assess the credibility of the evidence before we take decisions.' Photograph: Fabio Bucciarelli/AFP The US is considering a range of military responses to alleged evidence of chemical weapons use by Syria but remains cautious about significant intervention. Congressmen briefed by secretary of state John Kerry on Friday in Washington say the most likely option Kerry outlined would involve joining other countries in arming specific rebel groups. The imposition of a no-fly zone is also being considered but is deemed unattractive by the administration because chemical weapons do not require aircraft to be used - and because the high quality of Syrian air defences would put US lives at risk. There was also discussion of special forces use and specially designed ordnance designed to safely incinerate chemical weapons facilities, but there was scepticism that either would address the problem, which is thought to be widely dispersed. Kerry briefly took questions from politicians in the House of Representatives, but there has been a marked stepping down of rhetoric on Capitol Hill, even among more hawkish members of Congress. "There was about as much appetite in the room for getting involved as I have when I leave an all-you-can-eat restaurant," said Brad Sherman of California, who has previously sponsored a bill calling on Obama to arm rebels with anti-aircraft weapons. Other congressmen who discussed the briefing afterwards with reporters said they had more confidence now that the evidence pointing to chemical weapons use was strong, but not conclusive. "There is enough evidence to pass muster in a civil court, but I can't say it has been proved beyond all reasonable doubt," added Sherman. It remains unclear whether the US has independently established a clear "chain of custody" for soil samples and other evidence pointing to chemical use that may have been passed on by rebel groups. White House spokesman Jay Carney said there was still work to be done before weighing US options. We need to assess the credibility of the evidence before we take decisions on what action to take. On Thursday, the White House and other top Obama administration officials said that US intelligence had concluded with "varying degrees of confidence" that the Syrian government has twice used chemical weapons in its civil war. Two Syrian officials denied on Friday that government forces had used chemical weapons against rebels. In the Syrian capital, a government official said President Bashar al-Assad's military "did not and will not use chemical weapons, even if it had them." The official instead accused opposition forces of using them in a March attack on the village of Khan al-Assad outside of the northern city of Aleppo. Both sides have accused each other of the deadly attack. British prime minister David Cameron described the evidence of chemical weapons use as "limited but growing" and played down any suggestion British troops could be deployed in Syria as a consequence, saying only that it represented a red line for the international community "to do more." Speaking to the BBC, the prime minister echoed the White House's caution over the evidence so far, saying the UK government would not to make the mistake of "rushing into print" and would work to verify the evidence with its allies. It is very disturbing what we are seeing. It's limited evidence, but there's growing evidence that we have seen of the use of chemical weapons, probably by the regime," Cameron said. It is extremely serious. This is a war crime and we should take it very seriously. He also stopped well short of suggesting that confirmation of chemical weapons use would necessarily be a trigger for military action. I think what President Obama said was absolutely right - that this should form for the international community a red line for us to do more. There has been speculation that western special forces could be sent in to secure chemical weapons stockpiles in Syria, but Cameron said he could not envisage British troops on the ground. "I don't want to see that, and I don't think that is likely to happen, but I think we can step up the pressure on the regime, work with our partners, work with the opposition in order to bring about the right outcome," he said. British officials say there is evidence of sarin use in at least three incidents: at Khan al-Assal, in Homs and near Damascus. In the Khan al-Assal incident on March 19, the Syrian government and the rebels claimed that chemical agents had been used against them. British officials say that Syrian army troops appear to have been affected in that incident but suggest it was either case of "friendly fire," a projectile going astray, or a deliberate attempt to implicate the rebels. Chemical weapons experts have mostly reacted with caution over the claims. Referring to video footage purported to show victims foaming at the mouth, Richard Guthrie, a British chemical weapons expert and former head of the Chemical and Biological Warfare Project of the Stockholm International Peace Research Institute (Sipri), said: "That 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." Jean-Pascal Zanders, another 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. Some press reports of the sarin attack on the Tokyo metro system in March 1995 do refer to some victims foaming at the mouth. Experts also said that evidence in the form of physical samples, of soil or human tissue, would be of little use without a clear "chain of custody" between the site of an alleged attack at the laboratory where it was analysed. According to a report by McClatchy in the US, the soil sample examined by American experts is "minuscule" and contains a byproduct of sarin that could also be a byproduct of fertiliser production. The UN has launched an investigation in cooperation 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, and some have returned to their home countries. Additional reporting by Associated Press in Damascus