Syria conflict: BBC shown 'signs of chemical attack' 16 May 2013 Last updated at 08:51 The BBC has been shown evidence apparently corroborating reports of a chemical attack in Syria last month. A BBC correspondent who visited the northern town of Saraqeb was told by eyewitnesses that government helicopters had dropped at least two devices containing poisonous gas. The government has vehemently denied claims it has used chemical agents. The US has warned that such a development would be a "red line" for possible intervention. However, President Barack Obama has said the current intelligence on possible chemical weapon usage did not constitute sufficient proof. Suffocating smell In April, Saraqeb, a town south-west of Aleppo, came under artillery bombardment from government positions. Doctors at the local hospital told the BBC they had admitted eight people suffering from breathing problems. Some were vomiting and others had constricted pupils, they said. One woman, Maryam Khatib, later died. A number of videos passed to the BBC appear to support these claims, but it is impossible to independently verify them. Mrs Khatib's son Mohammed had rushed to the scene to help his mother and was also injured in the attack. Syria's chemical weapons The CIA believes Syria has had a chemical weapons programme "for years and already has a stockpile of CW agents which can be delivered by aircraft, ballistic missile, and artillery rockets" Syria is believed to possess mustard gas and sarin, a highly toxic nerve agent The CIA also believes that Syria has attempted to develop more toxic and more persistent nerve agents, such as VX gas A report citing Turkish, Arab and Western intelligence agencies put Syria's stockpile at approximately 1,000 tonnes of chemical weapons, stored in 50 towns and cities Syria has not signed the Chemical Weapons Convention (CWC) or ratified the Biological and Toxin Weapons Convention (BTWC) It was a horrible, suffocating smell. You couldn't breathe at all. You'd feel like you were dead. You couldn't even see. I couldn't see anything for three or four days," Mr Khatib told the BBC. A doctor who treated Mrs Khatib said her symptoms corresponded to organophosphate poisoning and that samples had been sent for testing. Hamish de Bretton-Gordon, a former commanding officer at the UK's Joint Chemical Biological Radiological Nuclear Regiment, said the testimony and evidence from Saraqeb was "strong, albeit incomplete." In Saraqeb and other similar events in Syria in recent weeks, "people have got ill and died and their symptoms are what we would expect to see from a nerve type of agent, be it sarin or be it organophosphate," Mr de Bretton-Gordon said. Mr de Bretton-Gordon has not visited the site or tested any of the alleged evidence but was given full access to the material gathered by the BBC. Both the US and UK have spoken of growing evidence that the Syrian government has used chemical weapons. Rebel fighters have also been accused of using them. They also have denied this. A 15-strong UN team headed by a Swedish scientist, Ake Sellstrom, has been assembled to investigate the claims. However, the Syrian government has refused the team access. Syrian officials have been quoted as saying they want the team to look into the incident in Khan al-Assal, but the team has requested unconditional access with the right to inquire into all credible allegations. The UN says estimates that the two-year-old conflict has left at least 80,000 people dead.