While the US Secretary of State, John Kerry, has been hailing the success of the US-led coalition's fight against the Islamic State (Isis) militants, Syria's President Bashar al-Assad thinks the strikes are ineffective.

Speaking to Paris Match magazine, Assad who is not a part of the coalition said, as reported by The Independent: "You can't end terrorism with aerial strikes. Troops on the ground that know the land and can react are essential.

"That is why there haven't been any tangible results in the two months of strikes led by the coalition."

Meanwhile, world leaders from 60 nations met at the North Atlantic Treaty Organization (NATO) headquarters in Brussels on Wednesday to discuss the progress of the US-led coalition's air strikes against IS, which started in August.

John Kerry, promising to continue the campaign for "as long as it takes", said that while the fight to defeat IS might take several years, the air strikes have been successful by targeting key IS leaders.

"Our commitment will be measured most likely in years but our efforts are already having a significant impact. The roughly 1,000 coalition air missions we have flown have reduced [IS's] leadership and inflicted damage on its logistical and operational capabilities," said Kerry.

Foreign leaders also assessed the impact of the Syria conflict at home. An estimated 3,000 Europeans have travelled to Syria to join the IS militants so far.

In a statement released by the coalition, it was revealed that future efforts will focus on reducing the outflow of locals joining the IS militants and a strategy to block IS financing.

Strategies to bring down the IS propaganda of using social media to recruit fighters were also discussed as a priority.

Islamic State militants are reportedly selling oil, stolen from captured oil fields, at less than half the global market rate, on the black market, earning themselves over £1 million per day, experts in the Middle East have reported.