They have been accused of fostering hatred between Syria's religious groups, pitching the Sunni Muslim majority against the president's Shia Alawite sect.

Such divisions have led both sides to commit atrocities, torn communities apart and dimmed hopes of peace.

They have also allowed the jihadist groups Islamic State (IS) and al-Qaeda to flourish.

Syria's Kurds, who want the right of self-government but have not fought Mr Assad's forces, have added another dimension to the conflict.

Who's involved?

Image copyrightREUTERS

The government's key supporters are Russia and Iran, while the US, Turkey and Saudi Arabia back the rebels.

Russia - which already had military bases in Syria - launched an air campaign in support of Mr Assad in 2015 that has been crucial in turning the tide of the war in the government's favour.

The Russian military says its strikes only target "terrorists" but activists say they regularly kill mainstream rebels and civilians.

Iran is believed to have deployed hundreds of troops and spent billions of dollars to help Mr Assad.