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.