As U.S. forces move within striking distance of Syria, and United Nations inspectors try to collect evidence to prove or refute reports of chemical weapon attacks in the outskirts of Damascus last week that reportedly killed hundreds or civilians, here’s a look at the weapons that might have been used in the attacks.

Q. What kind of chemical weapons does Syria possess?

Bashar Assad's regime is thought to hold hundreds of tonnes of mustard gas, sarin and VX gas – the most lethal of all chemical weapons. All are banned under the Chemical Weapons Convention, which has been signed by nearly all the world’s countries.

Q: What kind of chemical weapons were used in Syria?

A: It's not clear. UN experts have taken hair and blood samples from the victims, but an agent hasn’t yet been identified. But it’s likely chemical weapons called nerve agents were used, given the symptoms that the victims exhibited. Nerve agents include VX, taubun, and sarin gas. Earlier this year, France said its labs tested Syrian soil samples and confirmed that sarin was used "multiple times" in Syria.

Q: How do nerve agents work?

Most nerve agents were designed as pesticides until their chemical weapon uses were discovered. The gases work by blocking transmissions of nerve cell, including the nerves needed to breathe. They also interfere with neurotransmitters that act as “off switches” for glands and muscles causing them to become overstimulated and convulse.

Q: What are the symptoms reported in the Syrian victims?

A: The humanitarian organization Doctors Without Borders, which supports three hospitals in the eastern Damascus region, reported receiving roughly 3,600 patients over the course of less than three hours on Aug. 21. All had "neurotoxic symptoms." Of those, 355 died, the group said.

Many of the patients arrived with convulsions, excessive saliva, pinpoint pupils, blurred vision and respiratory distress. The pinpoint pupils are the telltale sign that the weapon used was likely a nerve agent and not another chemical agent, such as mustard or chlorine gas.

Q. How do nerve agents kill?

Dr. Anas Alkassem, a Canadian doctor who has been traveling to Syria to offer medical aid for two years, says the first symptom is a tight feeling in the chest. “It starts with difficulty to breathe, they gasp for air,” he told CTV’s Canada AM. “They have paralysis in their chest muscles. Then after that they’re going to have convulsions and twitching and it ends with coma.”

Depending on the dosage the victims receive, death by respiratory failure can occur within a matter of minutes or several hours.

Q. How is the gas dispersed?

Nerve agents such as sarin gas are typically stored in liquid form and can released into the air through small bombs or projectiles. The gas evaporates into a vapour and can then enter the body through the eyes, nose and mouth.

Sarin also mixes easily with water and can be used to poison water supplies or food; even touching sarin-contaminated water can lead to poisoning. Dr. Alkassem says it doesn’t take much of the chemicals to cause an effect: “You just need 0.5 mg exposure to this gas to have significant symptoms,” he said.

Q: Is there a treatment?

If an attack is spotted early, there is a dual chemical injection shot that can be used; many in the military carry this shot with them. But Dr. Alkassem says it’s harder to treat civilians, who can arrive for treatment hours after an attack.

“We’re not used to these weapons and so we don’t know how to handle the patients,” he said.

Nevertheless, he said the first priority is to stop the spread of the chemicals, which can cling to hair and clothes and continue to contaminate others.

“You cannot take them right away into the hospitals. So we provide them with tents outside the hospitals for decontamination. We wash them first because otherwise you’re exposing the medical staff (to the gas as well),” he said.

Then, the best that doctors can do is manage the symptoms.

Q. How are nerve agents identified after an attack?

Most nerve agent gases disperse and evaporate quickly making an identification through air samples difficult. But the gases do leave a residue in the area where they were used and can thus be detected if an investigation is begun within a matter of days. Members of a doomsday cult in Japan used sarin gas in two attacks in 1994 and 1995 and it took police just three hours to confirm the use of the gas through residue testing.

The gas also leaves traces in the body and can be detected in blood, urine or hair samples. But detection becomes difficult within just a few weeks, when the level of the chemical may no longer be measurable in a victim's body.

With files from The Associated Press