As the Syrian crisis enters its fifth year, here are some of the key events since the uprising against President Bashar Assad began:

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.

April 2011: Security forces raid a sit-in in Syria's third-largest city, Homs, where thousands of people tried to create the mood of Cairo's Tahrir Square, the epicenter of protests against Egypt's autocrat Hosni Mubarak.

June 2011: Police and soldiers in Jisr al-Shughour in northeastern Syria join protesters they were ordered to shoot, and the uprising claims control of a town for the first time. Elite government troops, tanks and helicopters retake the town within days.

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

July 2012: A bombing at the Syrian national security building in Damascus during a high-level government crisis meeting kills four top officials, including Assad's brother-in-law and the defence minister.

Summer 2012: Fighting spreads to Aleppo, Syria's largest city and its former commercial capital. Over time, rebels seize control of about half of the city, but the battle there rages to this day, leaving much of Aleppo in ruins.

August 2012: Kofi Annan quits as UN-Arab League envoy after his attempts to broker a cease-fire failed. Obama says the use of chemical weapons in Syria would be a "red line" that would change his thinking about military action.

November 2012: The Syrian National Coalition is created, bringing together the main opposition factions. The umbrella group is hampered from the outset by infighting and accusations that its members are out-of-touch exiles.

March 2013: After advancing in the north, rebel forces capture Raqqa, a city of 500,000 people on the Euphrates River and the first major population centre controlled by the opposition. That month, the number of UN-registered Syrian refugees tops 1 million, half of them children.

May-June 2013: Backed by thousands of Lebanese Hezbollah fighters, Assad's forces re-capture the strategic town of Qusair from rebels, near the border with Lebanon.

June 2013: U.S. officials conclude that Assad's forces used chemical weapons against the opposition. Obama authorizes direct support for the rebels.

August-September 2013: A chemical weapons attack in the Damascus suburbs kills hundreds. Obama, blaming Assad's government, says the U.S. has a responsibility to respond and puts it up to a vote in Congress. Russia proposes instead that Syria give up its chemical weapons, averting military strikes.

September 2013: Around a dozen rebel groups abandon the Syrian National Coalition and reject its calls for a civil, democratic government. Seven of them later form their own alliance, the Islamic Front, intended to eventually create a state governed by Islamic law.

October 2013: Syria destroys its chemical weapons production equipment. The number of Syrian refugees registered with the UN tops 2 million.

January 2014: Infighting among rebels spreads, pitting a variety of Islamic groups and moderate factions against the al Qaeda-breakaway Islamic State of Iraq and the Levant.

February 2014: Two rounds of peace talks led by UN-Arab League mediator Lakhdar Brahimi in Geneva end without a breakthrough.

May 9: Rebels withdraw from the old quarter of the central city of Homs in a significant symbolic victory for the government, putting the area that had been under siege for more than a year firmly in government hands.

May 13: Brahimi resigns as UN-Arab League envoy to Syria, marking a second failure by the United Nations and Arab League to end the civil war.

June 3: Syrians in government areas vote in presidential elections. Assad, one of three candidates, overwhelmingly wins with 88.7 per cent.

June: The Islamic State group, as the Islamic State of Iraq and the Levant is now known, seizes large parts of northern and western Iraq. In control of around a third of Syria and Iraq, it declares a self-styled Islamic caliphate.

July 3: Islamic State group takes control of Syria's largest oil field, al-Omar, after fierce battles with the Nusra Front, al Qaeda's branch in Syria.

July 10: Italian-Swedish diplomat Staffan de Mistura is named new UN envoy to Syria.

Aug. 19: Islamic State militants release video of the beheading of American journalist James Foley, the first of five Westerners to be beheaded by the IS group.

Aug. 24: Islamic State fighters capture Tabqa military air base in northeastern Syria, eliminating the last government-held outpost in Raqqa province.

Sept. 23: U.S.-led coalition begins airstrikes against Islamic State group targets in Syria.

Mid-September: IS begins offensive to take Syrian Kurdish town of Kobani, on the Turkish border.

January 2015: UN estimates Syria's conflict has killed at least 220,000 people and uprooted nearly a third of the prewar population of 23 million from their homes.

Jan. 26: With the help of U.S.-led airstrikes, Kurdish fighters take control of Kobani.

Feb. 3: IS releases a video of captured Jordanian pilot Muath al-Kaseasbeh being burned to death in a cage, sparking outrage in Jordan, which launches new strikes targeting the militants.

Feb. 6: IS claims a Jordanian airstrike kills American hostage Kayla Jean Mueller. U.S. officials later confirm her death, but say it wasn't caused by a Jordanian airstrike.

Feb. 25-28: IS overruns several Christian villages in Syria's eastern Hassakeh province, taking at least 220 Assyrian Christians hostage.