As Syria’s war nears the start of its fourth year, Iran has stepped up support on the ground for President Bashar al-Assad, providing elite teams to gather intelligence and train troops, sources with knowledge of military movements say.

This further backing from Tehran, along with deliveries of munitions and equipment from Moscow, is helping to keep Assad in power at a time when neither his own forces nor opposition fighters have a decisive edge on the battlefield.

Assad’s forces have failed to capitalize fully on advances they made last summer with the help of Iran, his major backer in the region, and the Hezbollah fighters that Tehran backs and which have provided important battlefield support for Assad.