Cricitising the US-led coalition’s reliance on airstrikes, he said to The Sunday Times: “Ultimately you need a land army to achieve the objectives we’ve set ourselves - all air will do is destroy elements of Isis, it won’t achieve our strategic goal

"The only way to defeat Isis is to take back land they are occupying which means a conventional military operation."

On the other hand,new U.S.-led air strikes against Islamic State terrorists failed to stop ISIL from pressing their assault on a strategic Syrian town near the Turkish border on Saturday, hitting it with shell fire for the first time.

The coalition on Saturday conducted its first strikes against Islamic State targets in a besieged Kurdish area of Syria along the Turkish border, destroying two armored vehicles in an area that has been the subject of a weeklong onslaught by the Islamic State.

The action around Kobani, where at least 150,000 refugees have crossed into Turkey, appeared to signify the opening of a new front for American airstrikes in Syria, and came on a day when several other strikes took place in Raqqa, the de facto headquarters of the Islamic State’s forces, and other sites in the eastern part of the country.

Since capturing swathes of territory in both Syria and Iraq, ISIL has proclaimed an Islamic "caliphate", beheaded many and ordered Shi'ites and non-Muslims to convert or die. Its rise has prompted President Barack Obama to order U.S. forces back into Iraq, which they left in 2011, and to go into action over Syria for the first time.

The U.S. military has been carrying out strikes in Iraq since Aug. 8 and in Syria, with the help of Arab allies, since Tuesday, in a campaign it says is aimed at "degrading and destroying" the militants.