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.

Thousands of Shia Muslim militiamen armed, trained and financed by Iran - mostly fromLebanon's Hezbollah movement, but also Iraq, Afghanistan and Yemen - have also fought alongside the Syrian army.

The US, UK, France and other Western countries have provided varying degrees of support for what they consider "moderate" rebels.

A global coalition they lead has also carried out air strikes on IS militants in Syria since 2014 and helped an alliance of Kurdish and Arab militias called the Syrian Democratic Forces(SDF) capture territory from the jihadists.

Turkey has long supported the rebels but it has focused on using them to contain the Kurdish militia that dominates the SDF, accusing it of being an extension of a banned Kurdish rebel group in Turkey.

Saudi Arabia, which is keen to counter Iranian influence, has also armed and financed the rebels.

Israel, meanwhile, has been so concerned by shipments of Iranian weapons to Hezbollah in Syria that it has conducted air strikes in an attempt to thwart them.