In recent months, the so-called Islamic State group has captured territory straddling Afghanistan's eastern border with Pakistan - claiming it as a new province of the "caliphate" it has declared in parts of Syria and Iraq. The BBC's Justin Rowlatt spoke to one of the IS leaders in this remote region.

It feels very odd to be telephoning an IS commander. The organisation has cultivated its reputation for savagery and slaughter so carefully it is hard to imagine what sort of person would choose to be part of this.

Maybe I won't find out. Each time we call we get a message saying his phone is not connected to the network.

"The Taliban blow up the mobile phone transmitters," explains Abdullah Nizami, a colleague from the BBC's Afghan Service ruefully. "Coverage is terrible."

Nizami is from the wild mountainous provinces in the east of Afghanistan where IS has been taking territory.

He knows "Zandani" - whose nickname means "The Prisoner" - by reputation.

Zandani (above, centre) has fought for the Taliban since his teens, but switched allegiance last year to IS, or Daesh, as they tend to call it here in Afghanistan.

Media playback is unsupported on your device Media caption Authorities struggle to cope with threats from the Taliban and IS, as Justin Rowlatt reports

Lots of Taliban fighters in Nangarhar are disillusioned, Nizami tells me. The revelation last summer that its leader Mullah Omar had died two years earlier caused huge upheavals in the organisation. Mullah Omar founded the Taliban, and shaped it into a national movement.

Without Omar that cohesion has been disintegrating. The new leaders of the main body of the Taliban are almost exclusively from the south of Afghanistan. In a country where local and tribal allegiances are incredibly strong, that's left many Taliban members from other regions feeling disenfranchised and provided fertile ground for IS.

A group of a few dozen, mainly Pakistani, IS supporters have been assiduously recruiting disaffected Taliban members, particularly in the border region in the isolated eastern reaches of Afghanistan.

They offer money, guns and the opportunity to achieve salvation fighting a global jihad - an alluring combination in this impoverished part of what is already an extremely poor country. And one which Afghan intelligence reckons has attracted hundreds of local fighters.

We call Zandani again. The same recorded message from the phone company plays out.

Unlike other extremist movements, IS's ambition has always been to seize and hold territory. Its members often use the phrase "baqiya wa tatamadad" - lasting and expanding - though the depredations of the international assault have left its "caliphate" in Syria and Iraq on the defensive.

That is why its expansion in Afghanistan is so significant. The extent of its reach shifts as its fortunes change in battle. According to the Pentagon its goal is to establish a safe haven here, potentially a new stronghold from which to launch attacks across the world.

But the price of its ambitions has, initially at least, been paid by local people.

On a dusty patch of wasteland on the outskirts of the regional capital, Jalalabad, two of my BBC colleagues meet a group of about 40 refugees, just a few of the hundreds of families driven out of their homes by IS.

They tell stories of horrific violence.

Image caption Rahman Wali is now looking after his brother's children

Rahman Wali breaks down in tears as he identifies first his friends, and then his brother in what is, even by IS's standards, a particularly sadistic propaganda video. He stops the player, knowing all too well what is coming.

His brother, Rahman Gul, is led with nine other villagers to where 10 bombs have been buried. Each man is forced to sit on a bomb. The bombs are then detonated.

The refugees tell my colleagues that about 100 of their men are missing. These aren't rival militiamen or local politicians, just poor villagers trying to scratch a living in their ancestral lands.

I dial Zandani's number again. I have almost given up hope of him picking up but to my astonishment, this time he answers.

"Salaam Alaikum," I find myself spluttering. I feel faintly queasy. "Peace be upon you," seems a singularly inappropriate greeting for this man.

I ask the obvious questions - why did he join IS, what does he want to achieve, how can he justify IS's actions - and get the obvious answers.

Image caption IS has attacked government checkpoints within an hour's drive of Kabul

Jason Burke, an expert on Islamist militancy, identifies the internationalisation of the ideology of global jihad as the most significant development in Islamist extremism in recent decades. It means militants from Britain, America, Turkey, Indonesia, Burkina Faso, or the remotest regions of Afghanistan - wherever - all trot out the same predicable justifications for their violence, and the same spurious theology.

Zandani tells me the Taliban is too provincial for him - he wants to fight a global jihad. He cites passages from Islamic writings to excuse the murder and torture IS has carried out.

I expect him to be furious when I say that Muslim scholars would say his interpretations of the holy texts are an insult to Islam, but he remains calm, claiming that Mohammed's own actions justify beheadings and slaughter.

Zandani tells me he has been fighting in Achin, the district the refugees come from. He says he plans further attacks. "At the moment we exist in three districts," he boasts, "but we only fight in one. In the others we are waiting for orders from our leader, then we will fight."

I am amazed he doesn't seem to care that most of his victims are fellow Afghans - and fellow Muslims. Those who don't take up the IS leader, Abu Bakr al-Baghdadi's, call to arms are "unfit", he says, by way of explanation. In his mind that clearly means you can slaughter them at will.

It is one of the most disturbing telephone conversations I have ever had. Most shocking is how complacent and unconcerned Zandani is about the horror he and his fellow fighters are visiting upon their own people.

I think of the phrase the writer Hannah Arendt coined to describe the way the Nazis systematised killing to make it "normal". She called it "the banality of evil".

Another journalist working with the BBC had met Zandani in the mountains where he and his fighters are based. It was an instructive encounter.

They are a motley crew, belying the slick professionalism projected by the IS propaganda machine. One man struggles to reassemble his AK47; the pieces fall uselessly into his lap.

The individuals may be banal, but IS's ambitions in Afghanistan are not.

This isn't the first time a foreign militant organisation has established a base in the mountains of eastern Afghanistan. The Tora Bora cave complex is in the same province where IS now holds territory and Tora Bora is, of course, where Osama Bin Laden built his stronghold.

But let's be clear: IS in Afghanistan doesn't present anywhere near the level of danger to the world that al-Qaeda did back then

"An emergent threat", is how IS in Afghanistan is described by the experts at Resolute Support, the NATO-led training and advisory mission set up when the international combat mission in Afghanistan ended in December 2014. What they mean by that is: "We're pretty certain it can only operate locally at the moment, but we aren't taking any chances."

And that's got to be the right call. My impression was that there wasn't anything particularly exceptional about Zandani - except that he has signed up to a bloodthirsty ideology - but his ordinariness doesn't mean he isn't frightening.

After all, men like Zandani perpetrate terrible atrocities almost every week.

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