Thousands of refugees stranded on arid mountainsides are faced with the grim choice of death at the behest of the Islamic State (IS) or be consigned to their demise through dehydration if they choose to remain.

These unenvied people are the Yazidi Kurds, an ancient religious minority of whom between 10,000 and 40,000 are trapped on Mount Sinjar in northwestern Iraq.

IS - who have stormed through northern Iraq to create a "Caliphate" straddling the Iraqi-Syrian border - have already killed 500 Yazidi men and 40 children in their siege of Sinjar, the Yazidi's ancestral home 50 miles west of the captured city of Mosul, and forced 200,000 to flee the area in what the United Nations has deemed a "humanitarian tragedy".

The group were being protected by Kurdish peshmerga fighters who were overrun by the Islamic militants as the protection of a shared 650-mile border with IS begins to stretch Kurdish resources.

As with the expulsion of Assyrian Christians in IS' blitz through northern Iraq, little if any international attention is being paid to the unfolding humanitarian crisis in the region.

So, who are the Yazidis? And why is the barbaric IS, formerly known as Isis, chasing these people into the mountains and killing them at will?

'Devil worshippers'

The Yazidis are an ancient community, concentrated in Iraq but also found in Iran, Syria and Turkey, who practice a religion where outsiders cannot convert and believers are hugely secretive about the details of their rituals.

It is reported that the Yazidi are incredibly wary of strangers following centuries of persecution. A Yazidi parliamentarian told the Christian Science Monitor: "In our history, we have suffered 72 massacres. We are worried Sinjar could be a 73rd."

Population: Approximately 600,000

Found: Mainly in northern Iraq. Also in Turkey, Iran and Syria

Main belief: God and seven Angels guard the world

Language: Kurmanji Kurdish

Worship: Yezidism

What is known about the ethno-religious group is that its roots are based in Zoroastrianism, an ancient Persian religion made famous by Queen singer Freddy Mercury, with parts of Christianity and Islam.

It also believes that God and seven angels protect the world. According to the Economist, the religion states that one of these angels, named Malak Tawous and believed to be embodied by a peacock on Earth, was thrown out of paradise for refusing to bow to Adam.

Thus, fundamental Muslims, and therefore IS, view this figure as a "fallen angel" and consider Yazidis to be "devil worshippers" and apostates. Yazidis claim that their religion is the oldest in the world, more so than both Islam and Christianity.

The Yazidi population is approximately 600,000 - with the majority speaking Kurmanji Kurdish - and, traditionally, those within the group cannot marry an outsider.

Yazidi Iraq ISIS
Displaced families from the minority Yazidi sect, fleeing the violence in the Iraqi town of Sinjarl west of Mosul, take refuge at Dohuk province. Reuters

A question of existence

Some of the other beliefs in this guarded sect include, but are not limited to: the honouring of sacred trees; marriage is forbidden in April; they refuse to eat lettuce, pumpkins or gazelles; dark blue is considered "too holy"; and they believe in reincarnation.

They are known to pray three times a day, always in the direction of the sun, while their sacred pilgrimage takes them to Lalish in northern Iraq at the tomb of Abi ibn Musafir, who was believed to be a descendent of the Peacock Angel, Malak Tawous.

In reaction to their current plight, Prince Tahseen, "the world leader of the Yazidis", issued an appeal to Kurdish, Iraqi and world leaders for assistance.

UN agencies have offered the Iraqi government help with conducting humanitarian airdrops to those on the mountainside but this offer has not yet been accepted by Baghdad.

Under the Ottoman Empire, troops wilfully killed many Yazidis and, much later, their villages were destroyed by Saddam Hussein's forces during fighting with Kurds.

Yet, the march of IS through northern Iraq now presents an even greater existential challenge for the sect. Both humanitarian and military assistance are essential in order to ensure their survival.

The Yazidis are known to turn to mountains and caves in times of persecution. If this help is not forthcoming in the Sinjar mountains, this time may be their last.