Does Syria have WMDs?
The BBC has been shown evidence which claims to show a chemical attack took place 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.
Both the Syrian military and the rebels have denied they've 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 is not sufficient proof.
In April, Saraqeb, a town south-west of Aleppo, came under artillery fire 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 shrunken pupils, they said.
One woman, Maryam Khatib, later died.
Mrs Khatib's son Mohammed had rushed to the scene to help his mother and was also injured in the attack.
"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 number of videos passed to the BBC appear to support these claims, but it is impossible to independently verify them.
What are the claims?
There are claims that Syria's embattled President Bashar al-Assad was planning to use chemical agents if put under further pressure to step down.
The warning was made by Nawaf el-Fares, the most senior politician to turn his back on the government since the fighting started there in 2011.
Syria has never confirmed they have these types of weapons.
According to independent intelligence sources, Syria has the biggest stockpile of sarin, VX nerve gas and mustard gas, in the Middle East.
It's thought to possess several hundred tons of toxins as well as rockets and missiles that can be used to deliver them.
There are said to be stockpiles in areas of current conflict, such as Homs and Hama, that are guarded by government troops.
The fear is if the government falls it could either use or lose the weapons.
How destructive are chemical weapons?
Mustard gas is also referred to as a blistering agent. Exposure to it can lead to an eventual slow and painful death.
It was made in large amounts during World Wars I and II.
Former Iraqi President Saddam Hussein used mustard gas and other weapons in the Kurdish village of Halabja in 1988.
Sarin can be used and released in canisters and it doesn't remain in the air after an attack. A single drop can kill an adult.
In 1995 about 13 commuters were killed when a religious sect released Sarin on the Tokyo metro system. A further 1,100 people were injured.
VX nerve gas is one of the most toxic and rapidly acting of the known chemical agents.
Immediate exposure to it can lead to suffocation as it disrupts the nerves that controls breathing. It's 10 times more deadly than sarin.
We've heard claims of WMDs before, in the case of Iraq. Where's the proof with Syria?
There isn't any concrete proof.
Syria has never declared what its stocks might be as it didn't sign the Chemical Weapons Convention (CWC) which outlaws production of such weapons.
It has officially stated that while it supports a region-wide ban on WMDs, it cannot unilaterally renounce chemical weapons, for as long as Israel continues to pose a threat to its security.
Opposition activists haven't reported the use of chemical weapons in Syria.
What's the international response likely to be if these weapons are used?
If the government used chemical and biological weapons, it would be a violation of international law and the pressure would mount on Syria's allies Russia and China to take a tougher stance on the country.
Neighbours like Israel are worried if the government falls, these weapons may end up in the hands of militant groups in the region.
In 2007 Israel bombed an unidentified site in Syria which it believed had been a nuclear reactor under construction.
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