The Russian army has stepped up its use of reconnaissance to direct artillery fire in eastern Ukraine as defence experts said the next phase of the war could hinge on which side has more unmanned aerial vehicles.

The Ukrainian defence ministry said that Orlan-10 drones were being used to scout defensive positions as the battle intensifies over the flat plains of Donbas.

Orlan-10s cost an estimated £75,000 and are equipped with infrared cameras that can spot targets. They transmit video in real-time to Russian gunners over a cellular network.

Despite its 3.1m wingspan, the Orlan-10 flies at relatively low altitudes, making it vulnerable to attack. Ukrainian forces claimed to have shot down four over Donbas yesterday.

On the Ukrainian side, defence experts believe that