Western powers are preparing for possible strikes against Syria over the purported use of chemical weapons last week that activists and an aid group say killed more than 300 people.

The Syrian opposition blames President Bashar Assad's regime for the attack. The government denies the charges and instead accuses the rebels of staging the assault to try to implicate the regime.

As a team of United Nations experts investigates in the country, here's a look at some of the chemical weapons that experts believe are in the Syrian government's arsenal:

— NERVE AGENTS

The most toxic of the chemical weapons, nerve agents affect the nervous system and are hazardous in their liquid and gas states. They can be delivered in missiles, bombs, rockets, artillery shells and other large munitions.

The Syrian regime is believed to possess tabun, sarin and VX. Absorbed through the skin or inhaled, these agents can — within seconds or minutes depending on the dose — cause extreme runny nose and salivation, blurred vision, nausea, vomiting, diarrhea and convulsions. Death is generally caused by paralysis of the respiratory system, which causes the victim to suffocate.

Germany developed the first nerve agents before and during World War II, although the Nazis did not use them during the war. The only time nerve agents are believed to have been used on the battlefield was in the 1980s Iraq-Iran war.

— MUSTARD GAS

One of the best-known chemical weapons, mustard gas is a blister agent that attacks the eyes and skin, causing severe blisters. If inhaled, it can also damage the lungs and other organs. The gas does not cause immediate symptoms, which means those exposed to it can unknowingly take high dosages. While not usually lethal, exposure to mustard gas is generally debilitating.

Mustard gas was first used by the German army in World War I against British forces. Saddam Hussein was accused of using the gas in Iraq's war with Iran, as well as in his 1987-1988 crackdown on the country's Kurdish minority. The most notorious case was on the village of Halabja, which killed some 5,000 people in the deadliest chemical weapons attack ever against civilians.