Islamic State fighters tightened their siege on a town on Syria’s border with Turkey yesterday despite US led air strikes aimed at defeating the militants in both Syria and Iraq.

More than a month since Washington began striking Islamic State targets in Iraq, and four days since it extended the campaign into Syria, there are signs the fighters are lowering their profile in areas they control to become a harder target.

However, the air campaign has yet to halt their advance in Syria, where they have laid siege to a Kurdish town on the Turkish border, sending 140,000 refugees across the frontier since last week in the fastest flight of the three-and-a-half-year-old civil war.

Some Kurdish commanders have said the air campaign has made the militants’ advance even stronger, by prompting the militants to move armour out of positions in cities and send it to the front lines, where Western planes have yet to strike.

The main battle in northern Syria has been visible from across the frontier in Turkey. The boom of artillery and bursts of machinegun fire echoed across the border, and at least two shells hit a vineyard on the Turkish side, though there were no immediate reports of casualties inside Turkey.