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RAF fighter jets were heading for Iraq as an Islamic State warlord issued a blood-curdling warning that the world “has seen nothing yet”.

Britain despatched Tornado warplanes to beef up the aid mission for the tens of thousands of desperate Iraqis being besieged by the barbaric militants .

With Iraq facing a humanitarian disaster, the US ordered more air strikes and pledged to arm Kurdish fighters attempting to halt the advance of the Islamic State.

Haji Othman, a commander of the jihadists, responded and said: “You have seen nothing yet.

“This is just the beginning. So far we’ve used just a minimal part of the strength that we have at our disposal.

“You cannot imagine how strong we really are. We have immense power. You will be amazed.

"You won’t be able to resist us. We’ve never feared the Americans, not even in the past when we were much weaker.

“Why would we fear them now?

"We’ve defeated them before and we’ll defeat them again.

(Image: Reuters)

"May Allah curse the Americans and their allies. They’ll suffer an ugly death.”

He also said in the interview with an Italian newspaper that “non-believers” in northern Iraq must convert to Islam , pay a tax or face certain death.

The Pentagon has admitted that while the US air strikes have slowed the Islamic State they are unlikely to have substantially weakened the group it as it continues to slaughter innocent people.

Lieutenant General William Mayville Jr, a senior Pentagon official, said the blitzes had disrupted the militants’ advance towards Erbil, the capital of Iraqi Kurdistan, but had probably not made any impact on the group’s operations in other parts of Iraq or Syria.

The Islamic State has trapped tens of thousands of Yazidis – a sect labelled as devil worshippers by the militants – in the Sinjar mountains without food or water.

Women and children have reportedly been buried alive during the jihadists’ murderous onslaught.

But others have managed to flee the area – now known as Death Mountain – and have made it to relative safety in places such as Haseki over the border in Syria.

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Petrified families – many on foot, some crammed into cars and other dust-covered vehicles – have escaped after days trapped in the mountains.

The Tornado deployment takes Britain a step closer to military action and follows growing pressure on David Cameron to do more to help the Yazidis.

The jets will assist with surveillance after a UK aid drop for those stranded on Mount Sinjar had to be aborted overnight because it was feared that falling packages would injure the numerous refugees.

No 10 said: “We have decided to pre-position a small number of Tornados in the region so that they could, if required, use their excellent surveillance capability to gather better situational awareness to help with humanitarian effort.”

There were questions about who was co-ordinating the British response, with Mr Cameron and his deputy Nick Clegg out of the country and Downing Street unable to say who was the most senior Government figure still in the UK.

Former Tory defence minister Andrew Robathan said it was “no good just sending aid” and urged the Government to consider joining the US in air strikes to stop the advance of the jihadists.

(Image: Reuters)

Military top brass and senior Labour figures also said the Government had a duty to prevent a possible genocide by the militants.

Labour’s ex-Foreign Secretary Jack Straw said direct military action should be “actively considered”.

He added: “I hope that the British government are now accelerating their humanitarian intervention and assistance.

"It’s taking a bit of time to get going and these poor devils are dying at the moment.”

Former head of the Army, General Lord Dannatt, said: “The air strikes need to be stepped up and they need to be well targeted.

"It may need people on the ground to make sure the air strikes are accurate and therefore effective.”

Downing Street said it could also send arms to the Kurds but Foreign Secretary Philip Hammond said the UK’s focus was on the humanitarian effort.

Horrific videos of the Islamic State carrying out executions have sent shock waves around the globe.

There are fears the extremists – the world’s wealthiest jihadist group – have taken world leaders by total surprise, even though intelligence agencies have warned about the uprising for months.

In just three years the group has grown into a billion-pound network, raking in millions every month from stolen oil revenues, extortion, enforced taxation and smuggling.

(Image: Getty)

Its fighters – numbering tens of thousands, including hundreds of Brits – are the highest-paid foot soldier jihadists, earning up to £300 a month.

And their armoury is enormous, with a huge supply of weapons which includes battle tanks capable of smashing a building to the ground with one shot.

Iraq has lurched into a fresh political crisis as the president Fuad Masum tried to oust the prime minister Nouri al-Maliki.

Commentators said this was a bid to save the crumbling state.

But the Iraqi PM said he was still planning to stand for a third term.

Professor Peter Neumann, director of the International Centre for the Study of Radicalisation, said the turmoil has raised the chances of the Islamic State taking the capital Baghdad .

He said the danger of Iraq collapsing was now “higher than 50/50”.

We must put forces on the ground to stop such barbaric butchers

By Col Richard Kemp, Iraq war veteran

Iraq faces its own destruction, genocide, large-scale violent abuse and the persecution and slaughter of Christians, Yazidis and all minorities.

There can be no attempt to negotiate or apply diplomatic pressure on the barbaric butchers of the Islamic State.

They are interested only in mass murder, torture and spreading unmitigated fear to bring about their vision of an Islamic caliphate in the Middle East and the world.

They and their jihadist bedfellows, including core al-Qaeda terrorists, are the most dangerous imperialists of the 21st century. They are testing the strength of Middle Eastern governments and the West.

If they are unchecked they will rampage far and wide, well beyond Iraq and Syria. In our wider interests, they must be stopped.

We will not stop them by humanitarian airlifts but by attacking them with our American allies. This will have to go beyond air strikes to forces on the ground.

History has proved repeatedly that air strikes provide a great military advantage but do not work against a moveable and adaptable force such as the Islamic State unless they are followed up with engagement on the ground. We also have an obligation to save the Iraqis from this brutality.

The purpose of the 2003 invasion was to prevent Iraq from allying with al-Qaeda with the intolerable threat that would represent in the post-9/11 world. This situation, although different, presents the same threat.