Below are events that have led to charges by Britain, France and the United States that Syrian troops have used chemical weapons and by Russia that rebels have used them.
- July 23: The Syrian regime acknowledges for the first time that it has chemical weapons and threatens to use them in case of a Western military intervention, but never against its own people.
- August 20: US President Barack Obama says the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line".
- December 3: NATO warns Syria against using chemical weapons, as a US official says that Syria has begun mixing chemicals that could be used to make sarin gas, a deadly nerve agent.
- December 24: Russian Foreign Minister Sergei Lavrov says it would be "political suicide" for Syrian President Bashar al-Assad to use chemical weapons against the armed opposition, and says he does not believe he would.
- January 30: The Israeli air force hammers a military complex near Damascus suspected of holding chemical agents, saying it fears their transfer to Lebanese Shiite militant group Hezbollah, according to a US official.
- March 19: The government and rebel forces accuse each other of using chemical weapons for the first time.
- March 20: Obama warns the Damascus regime any use of chemical weapons against civilians would be a "game-changer".
- April 8: UN Secretary General Ban Ki-moon says a UN inspection team is in nearby Cyprus and ready to deploy. Syria rejects the mission as proposed by Ban.
- April 25: The White House says that Syria has probably used chemical weapons against rebel forces on a "small scale," while emphasising that US spy agencies are still not 100 percent sure.
- May 10: US Secretary of State John Kerry says there is "strong evidence" that the regime has used chemical weapons against rebels.
- May 18: Assad denies that his forces have used chemical weapons.
- June 4: UN investigators say they have "reasonable grounds" to believe that both sides in Syria have employed chemical weapons, on four occasions.
France says it "is clear" that Assad's regime is using sarin gas.
- June 13: The White House says a "red line" has been crossed and accuses the Syrian regime of using chemical weapons.
- June 14: The Kremlin says the US accusations are "unconvincing". Damascus calls them "lies".
Britain says it agrees with the US assessment. NATO says Damascus must let the UN investigate.
- July 9: Russia's ambassador to the UN says Moscow has proof that rebels used sarin gas in the northern town of Khan al-Assal on March 19. The White House says it has seen no such proof.
- July 23: The UN has been told of 13 alleged chemical weapons attacks in Syria, a top official says.
- July 24: Two senior UN chemicals weapons inspectors arrive in Damascus.