Timeline of chemical weapons attacks in Syria

The suspected gas attack in the Syrian town of Douma over the weekend has once again highlighted the use of chemical weapons in Syria's seven-year civil war.

Chemical weapons have killed hundreds of people since the start of the conflict, with the U.N. blaming four attacks on the Syrian government and a fifth on the Islamic State group.

The latest attack killed at least 40 people and has been widely blamed on the Syrian government, which has denied responsibility. President Donald Trump has said there will be a "big price to pay" for President Bashar Assad's government and anyone else found to be responsible.

The Organization for the Proliferation of Chemical Weapons says it will send a fact-finding mission to investigate the latest attack, following an invitation from the Syrian government.

The following is a timeline of chemical weapons attacks in Syria:

March 2011: Protests erupt in the city of Daraa over security forces' detention of a group of boys accused of painting anti-government graffiti on the walls of their school. On March 15, a protest is held in Damascus' Old City. On March 18, security forces open fire on a protest in Daraa, killing four people in what activists regard as the first deaths of the uprising. Demonstrations spread, as does the crackdown by Assad's forces.

Aug. 18, 2011: President Barack Obama calls on Assad to resign and orders Syrian government assets frozen.

Aug. 20, 2012: Obama says the use of chemical weapons would be a "red line" that would change his calculus on intervening in the civil war and have "enormous consequences."

March 19, 2013: The Syrian government and opposition trade accusations over a gas attack that killed some 26 people, including more than a dozen soldiers, in the town of Khan al-Assal in northern Syria. A U.N. investigation later finds that sarin nerve gas was used, but does not identify a culprit.

Aug. 21, 2013: Hundreds of people suffocate in rebel-held suburbs of the Syrian capital, with many suffering from convulsions, pinpoint pupils, and foaming at the mouth. U.N. investigators visit the sites and determine that ground-to-ground missiles loaded with sarin were fired on civilian areas while residents slept. The U.S. and others blame the Syrian government, the only party to the conflict known to have sarin gas.

Aug. 31, 2013: Obama says he will go to Congress for authorization to carry out punitive strikes against the Syrian government, but appears to lack the necessary support in the legislature.

Sept. 27, 2013: The U.N. Security Council orders Syria to account for and destroy its chemical weapons stockpile, following a surprise agreement between Washington and Moscow, averting U.S. strikes. The Security Council threatens to authorize the use of force in the event of non-compliance.

Oct. 14, 2013: Syria becomes a signatory to the Chemical Weapons Convention, prohibiting it from producing, stockpiling or using chemical weapons.

June 23, 2014: The Organization for the Prohibition of Chemical Weapons says it has removed the last of the Syrian government's chemical weapons. Syrian opposition officials maintain that the government's stocks were not fully accounted for, and that it retained supplies.

Aug. 7, 2015: The U.N. Security Council authorizes the OPCW and U.N. investigators to probe reports of chemical weapons use in Syria, as reports circulate of repeated chlorine gas attacks by government forces against civilians in opposition-held areas. Chlorine gas, though not as toxic as nerve agents, can be classified as a chemical weapon depending on its use.

Aug. 24, 2016: The joint OPCW-U.N. panel determines the Syrian government twice used helicopters to deploy chlorine gas against its opponents, in civilian areas in the northern Idlib province. A later report holds the government responsible for a third attack. The attacks occurred in 2014 and 2015. The panel also finds that the Islamic State group used mustard gas.

Feb. 28, 2017: Russia, a stalwart ally of the Syrian government, and China veto a U.N. Security Council resolution authorizing sanctions against the Syrian government for chemical weapons use.

April 4, 2017: More than 90 people are killed in a suspected nerve gas attack on the town of Khan Sheikhoun in the rebel-held Idlib province. Victims show signs of suffocation, convulsions, foaming at the mouth and pupil constriction. Witnesses say the attack was carried out by either Russian or Syrian Sukhoi jets. Moscow and Damascus deny responsibility.

April 4, 2017: Trump says the "heinous" actions of Assad's government are the direct result of Obama administration's "weakness and irresolution."

April 5, 2017: Trump says Assad's government has "crossed a lot of lines" with the suspected chemical attack in Syria.

April 6, 2017: The U.S. fires cruise missiles into Syria in retaliation for the Khan Sheikhoun attack, in the first direct American assault on the Syrian government.

Oct. 26, 2017: Experts from the U.N. and the chemical weapons watchdog blame the Syrian government for the attack on Khan Sheikhoun. The report supports the initial findings by the United States, France and Britain that a Syrian plane dropped a bomb with sarin on the town of Khan Sheikhoun.

April 7, 2018: Syrian activists, rescuers and medics say a poison gas attack on the rebel-held town of Douma near the capital has killed at least 40 people. The Syrian government and Russia reject the allegations, saying the purported evidence of a chemical weapons attack was fabricated.

April 9, 2018: Trump says he will decide on a U.S. response to the Douma attack "probably by the end of today."

April 10, 2018: Syria says it has invited the OPCW to send a fact-finding mission into the country, as government forces across Syria go on high alert in anticipation of a possible U.S. strike.