David Cameron has pledged to help the US “destroy” Isis’ so-called caliphate, which has spread across swathes of Iraq and Syria.

In his strongest signal that the UK could soon join US-led coalition air strikes in Syria, the Prime Minister said he wanted the country to “step up and do more”. However, he stressed he would need to receive parliamentary approval beforehand.

Asked to address the threat posed by Isis during an interview on US television network NBC, Mr Cameron said: "Look, we know that we have to defeat Isil, we have to destroy this caliphate, whether it is in Iraq or in Syria.

"That is a key part of defeating this terrorist scourge that we face.

"I want Britain to do more. I'll always have to take my parliament with me. We're talking and discussing at the moment, including with the opposition parties in Britain, what more we can do.

He went on: "But be it no doubt, we're committed to working with you to destroy the caliphate in both countries,“ he told the Meet The Press programme to be broadcast today.

Meanwhile, a former head of the armed forces has warned the existing strategy against Isis is "firing up" the situation.

Lord Richards said political leaders have failed to grasp the scale of response required to take on the extremists in their Iraq and Syria strongholds, and only have a limited time to improve the existing approach which he described as "woefully insufficient".

He told the BBC's Andrew Marr Show: "The current strategy won't work in the time I think we've got available.

"I worry that - what we call in the Army 'dribbling' instead of 'clouting' - if we dribble, which is really rather what we are doing at the moment, it is simply firing up the problem rather than dealing with it.

Iraqi Kurdish Peshmerga fighters hold an Isis flag upside down after re-claimed a town in Iraq, where Britian as taken part in US-led airstrikes. (Getty Images)

Mr Cameron appeared on the programme prior to a speech in which he is expected to set out how the Government plans to tackle the threat of fundamentalist terrorism until the end of the decade.

In what aides have described as a “significant” speech, the Prime Minister will focus on deterring those attracted by the supposed “glamour” of joining Isis in Iraq or Syria by warning the group will simply use them as “cannon fodder”.

Warning young people against travelling to join the group also known as Isil, he said: "You won't be some valued member of a movement. You are cannon fodder for them. They will use you."

"If you are a boy, they will brainwash you, strap bombs to your body and blow you up. If you are a girl, they will enslave and abuse you. That is the sick and brutal reality of Isil.

"We must de-glamorise the extremist cause, especially Isil. This isn't a pioneering movement, it is vicious, brutal, fundamentally abhorrent."

Power struggle: Isis areas gained and lost Show all 1 1 /1 Power struggle: Isis areas gained and lost Power struggle: Isis areas gained and lost Isis areas gained and lost (Maximise window for full graphic)

The wide-ranging speech will also see Mr Cameron reject complaints that new rules obliging schools to prevent radicalisation amount to spying on Muslim children, and appeal to all faiths to “support the British way of life”.

The Prime Minister’s recent invite Labour’s interim leader Harriet Harman to attend a National Security Council briefing on the threat of fundamentalist terrorism last week has been regarded as a step towards a parliamentary vote to extend air strikes later in the year.

The decision was further complicated by the revelation that RAF pilots were allowed to take part in airstrikes over Syria, in the face of a parliamentary vote against such a move.