BARACK Obama has made it clear the West will defeat the "extremist nihilism" of the Islamic State as he toughened up his stance after claims of a lack of leadership by the US president against the terror threat.

Speaking at the end of the Nato summit, Mr Obama said: "We are going to achieve our goal. We are going to degrade and ultimately defeat ISIL (IS), the same way that we have gone after al Qaeda."

It was under his presidency that US forces found and killed Osama bin Laden.

"You initially push them back, you systematically degrade their capabilities, you narrow their scope of action, you slowly shrink the space, the territory that they may control, you take out their leadership, and over time they are not able to conduct the same kinds of terrorist attacks as they once could," he told reporters.

In his final message from the summit, Prime Minister David Cameron also stressed how the West must act "carefully and methodically" to deal with the IS threat amid a growing clamour for Britain to join US air strikes against the terrorists controlling large swathes of Iraq and Syria.

But Mr Cameron insisted Britain was not yet at the stage where it was ready to take offensive military action and called for a "comprehensive plan" involving nations in the region.

Earlier, US secretary of state John Kerry appeared to try to up the tempo, calling for agreement on an international plan for dealing with IS by the time of the general meeting United Nations General Assembly in New York later this month.

Mr Kerry and British Foreign Secretary Philip Hammond, jointly chaired a meeting of foreign and defence ministers from what the US called a "core coalition" of allies to discuss the way forward.

Earlier Mr Hammond made clear the UK Government would not be deterred from launching air strikes against IS by threats to the life of a British hostage they are holding.

IS militants - also referred to as Isil or Isis - have threatened to kill Scottish aid worker David Haines who they are believed to be holding in Syria.

They have already beheaded two American journalists, posting the evidence online in gruesome videos featuring a masked jihadist with a British accent.

"We will do everything we can to protect this individual (Mr Haines) but we have to be clear about the nature of the organisation that we are dealing with," explained Mr Hammond.

"They are utterly brutal, they are ideologically driven - almost pathological in the way that they behave. We cannot allow our strategy to be driven by their behaviour. We have to approach the challenge of Isil with focus purely on what is in Britain's best interests; protecting our citizens and our security from the threat that Isil poses from foreign fighters, from potential attacks on our soil, destabilisation of the region.

"We will not be diverted from doing what is right by the threats from this organisation," he said.