An allegedly exclusive interview with the Chechen military commander of the Islamic State has been denounced as a hoax by social media users.

In the interview with an Australian radio station, Omar al-Shishani allegedly calls Yazidi Kurds "the filthiest creatures on Earth" and says the Islamic State's "caliphate" will grow "until the rule of Allah is universal".

But doubts have been raised as there is previously no evidence of Shishani speaking fluent English.

Born and raised in Chechnya, the military commander is known to speak fluent Chechen and broken Russian and Arabic.

The man, who claimed to be the group's military chief, spoke candidly in English and Arabic about the Islamic State, the treatment of Christians and Yazidis, and the new front against the Iraqi Kurds in a 10-minute Skype interview with Tom Elliott, a presenter from Melbourne's number-one radio station, 3aw.

"We want it [Caliphate] insha'Allah to be without borders and only fronts. Only places where we fight until the law of Allah is above every law," he said.

According to 3aw, Professor Greg Barton, the international director of the Global Terrorism Research Centre, said the interview was consistent with the research his centre had conducted into the Islamic State.

But the expert also told The Australian newspaper that "it's highly unlikely that the voice in the interview is Omar al-Shishani".

There is also little evidence of the first-ever Western interview given by an Islamic State commander on the group's social media and online outlets.

One social media user said: "It's odd that he would use something so easily traceable as Skype."

Another said: "Thats probably not Abu Omar al-Shishani. He doesn't have a Chechen accent and the way he says "my friend" after every sentence leads me to believe this is some fanboy or kid putting on an eastern European accent."

In other video footage of Shishani speaking - which can be viewed below - his voice is much higher in comparison to the deep voice heard on the 3aw radio interview.

It is therefore likely that this interview is not of Shishani but a hoax or, at most, another Islamic State militant.

Elliott stands by the interview, saying it could be another fighter within the group.

"Prior to putting him on air, we did everything we could to establish his identity as genuine," he said.

"We stand by our belief that the man... was an Islamic militant fighting for the violent overthrow of the Iraqi government and the establishment of a fundamentalist caliphate. But you just never know these days, do you?"

'No place for these devil worshippers'

In the 'interview', Elliott questioned 'Shishani' on reports of beheadings and crucifixions in the Syrian city of Raqqa, which the Islamic State has claimed as the capital of its caliphate, and in Mosul.

"Yes. There have been crucifixions, there have been beheadings. But not against Christians because they are Christians. You are acting silly my friend," said 'Shishani'.

"Everybody who makes trouble in our land will get crucified," he warned.

The interviewee was more scathing about the Yazidis, who he said would not be allowed into the caliphate, unlike Jews or Christians who can "pay protection money" to live alongside members of the Islamic State.

"They [the Yazidis] are the filthiest of creatures on Earth. We will fight them. We will take their women, we will take their children. They have to come to Islam or they will get wiped out. There is no place for these devil worshippers."

Isis boy
During the interview the speaker said Khaled Sharrouf, father of the boy pictured holding a severed head, was a "good man". Twitter

On what is next for the Islamic State, 'Shishani' said the militants are focusing on "the fronts against Kurdish people. We have a big issue with these people".

After dealing with the Kurds, the man purporting to be 'Shishani' said the next focus would be the Iraqi Shia security forces.

"We will push them all the way to Tehran and when we are finished in Iraq, we will come after them in Tehran. If they hide in Australia, we will come to Australia!"

Shishani is considered one of the most influential leaders of the Syrian opposition, after a series of assaults on military bases near Aleppo and as a mastermind of the capture of the northern Iraqi town of Mosul.

The interview has not been independently verified.

Listen to the alleged interview between 3aw host Tom Elliott and Omar al-Shishani below.