Q&A: Sarin nerve agent
The United States say it has evidence that Syrian government forces used the chemical agent sarin in attacks on the outskirts of Damascus last month. BBC News looks at what it is and when it has been used.
What is sarin?
Sarin is one of several substances called nerve agents, with others including VX, tabun and soman.
Sarin is a clear, colourless and tasteless liquid which quickly turns to vapour.
Nerve agents are extremely deadly - much more so than strong poisons such as cyanide.
Amounts small enough to fit on a pinhead can prove fatal in under two minutes.
Death usually occurs within a quarter of an hour of exposure to its vaporous form. Exposure can also occur if the skin is exposed to the agent in liquid form.
Someone who has been exposed to sarin can develop an array of symptoms, including: a runny nose; drooling; painful watery eyes and blurred vision; breathing problems; nausea, vomiting; diarrhoea; and convulsions.
In extreme cases, convulsions and respiratory failure are followed by death.
There are antidotes to sarin - atropine and pralidoxime - but these must be administered immediately to be effective.
The victim should be taken to a safe area and all of their clothes removed to avoid further contamination. Their eyes should be flushed out with water and their skin and hair cleaned with an absorbent powder such as talcum or flour or, failing that, soapy water. Showering should be avoided as this may spread the agent.
Medical staff must take care to protect themselves by wearing gloves, a mask and goggles if possible.
When has it been used?
Sarin was invented in Germany in the 1930s but was not used in combat during World War II.
After the war most major powers developed nerve gas, with the British inventing another kind, VX.
Sarin was among the agents used by the Iraqi government when it killed 5,000 Iraqi Kurds in Halabja in 1988.
Perforated bags of liquid sarin were left in the Tokyo underground in the 1995 attack by the Japanese Aum Shinrikyo cult. Twelve people died.
US Secretary of State John Kerry said samples from hair and blood gathered after the 21 August attack on the outskirts of Damascus "tested positive for signatures of sarin".
The US has previously spoken of similar evidence of sarin use in other attacks in the conflict in Syria.
How would Syria have acquired sarin?
Syria is believed to have started producing sarin in the 1980s.
US officials claimed Syria started converting several pesticide plants to the production of sarin in 1988, according to a Defence and Foreign Affairs report.