ISIS has escalated its scorched earth campaign in Syria, setting gas fields west of Palmyra ablaze as the army advanced on them.

After weeks of fighting to secure their outpost at the T4 airfield, troops broke through towards the ruined ancient Roman city yesterday morning, but not before retreating Isis fighters blew up gas wells in the area the night before.

In December, Palmyra fell for a second time to a surprise offensive by 5,000 Isis zealots — with Russia alleging that the US-led bombing coalition had eased pressure on the death cult in Raqqa and allowed militants to escape to the norther Iraqi city of Mosul.