Shekhan, Iraq - Khalaf Qassim looks visibly distressed, sitting in the courtyard of the home of the spiritual leader of one of Iraq's most ancient religions, in this usually quiet town about 50km north of Mosul.

Qassim is one of tens of thousands of people who have fled the predominantly Yazidi area of Sinjar, in Iraq's western Nineveh province, after Islamic State fighters overtook the area on Sunday.

"Where are you going to go? I swear [to] God I will cut you into pieces… We are coming for you, you pig, you enemy of God," read a text message that Qassim showed Al Jazeera on his cell phone. He said the message came from a member of the Islamic State group on Friday. "Didn't I tell you yesterday to come and repent," it continued.

Qassim used to live in Bari village along the border with Syria, and previously was a member of the security forces in the nearby town of Sinjar. When Islamic State fighters took over the area on Sunday, he said he and fellow villagers - members of the ancient Mesopotamian Yazidi faith - left immediately.

"We are known on both sides of the border [in Iraq and Syria]," said Qassim. "My uncle and I left behind 3,000 tonnes of wheat and barley."

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Yazidis follow a religion that predates Islam and Christianity and originated in Mesopotamia, which encompassed modern-day Iraq and parts of neighbouring states. Yazidism, or Ezdayeti, as followers of the religion call it, shares elements with Abrahamic religions, but retains many individual practices and beliefs.

Today, most Yazidis live in northern Iraq and Syria, but followers of the religion have also scattered across some former Soviet republics such as Georgia. In Iraq, most Yazidis live in the province of Nineveh, in towns like Sinjar, Shekhan and Bashiqa, and most speak the Kurmanji dialect of Kurdish.

Members of the religious minority are viewed as infidels by Islamic State fighters, who began to take over large swaths of land in Iraq in early June.

Extremist forces have targeted Yazidis in the past: In August 2007, around 500 Yazidis died in coordinated attacks on villages near Sinjar.

Yazidi sources told Al Jazeera that Islamic State fighters have asked the remaining Yazidis under their control to convert to Islam or face death. Islamic State-affiliated social media accounts , meanwhile, have posted images of summary executions of individuals in Sinjar and surrounding areas.

Multiple Yazidi sources told Al Jazeera that Islamic State fighters have taken dozens of women hostage and moved them to the nearby town of Talafar, which has been under the group's control since June. These claims could not be independently verified as journalists cannot enter Islamic State-controlled areas.

The courtyard where Qassim and a dozen other men have gathered now resembles a crisis management centre. Samir Baba Sheikh, the son of the Yazidi spiritual leader, who often acts as his representative, was frantically calling senior officials in the Kurdish government and foreign diplomats on Monday.

"We call on the world, especially the Kurdish government and leadership to act, to do something about this situation," Baba Sheikh told Al Jazeera.

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According to the United Nations Office for the Coordination of Humanitarian Affairs , between 35,000-50,000 Yazidi civilians are stuck in Sinjar, surrounded by Islamic State fighters on all sides. People are rapidly running out of food and water supplies, and the UN Special Representative for Iraq, Nickolay Mladenov, warned that " a humanitarian tragedy is unfolding in Sinjar ".

Members of the group trapped in the mountains told Al Jazeera that as many as 30 people have starved to death so far, the majority of whom are children, while some Yazidi men - armed with light weapons - are protecting the routes that Islamic State fighters could use to get to the civilians. These reports could not be independently verified by Al Jazeera.

We cannot get out of here; we cannot move. It's all [Islamic State] fighters around us. If we move down they will kill us. - Qassim Barakat, Yazidi-Iraqi trapped in Sinjar mountains

"We cannot get out of here; we cannot move. It's all [Islamic State] fighters around us. If we move down they will kill us," Qassim Barakat, who is stuck in the mountain with 15 members of his extended family, told Al Jazeera by phone. "We want the world to help us. We want the United Nations to help us."

"We are here without any shelter. Everyone, children and women, are laying down under the sun," said Shakir Hassan, who said he escaped to the mountain from the nearby Gir Aziz district with hundreds of other families. "We call on any power who can to come save us."

Many of the families who fled to the mountain were either those who did not have a car to drive to the safer areas in Peshmergas' hands or mistakenly thought that ISIS would not be able to stay in Sinjar very long.

On Tuesday, Yazidi sources told Al Jazeera that helicopters - thought to belong to the Iraqi military - had dropped food and water to those caught in the Sinjar Mountain. But some of the supplies were rendered useless as they flew at a high altitude to avoid being shot down by Islamic State fighters.

The roads from Shekhan to Zumar, another area that Islamic State fighters captured on Saturday, are full of displaced families camping on both sides of the road without any protective shelter. Others are now living in two camps for internally-displaced persons in Sharia and Khanik.

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As the crisis continues, many people are angry with the Kurdish Peshmerga forces for not resisting the Islamic State group's advance. "The Peshmerga forces in our area were weak," Shakir said. "They did not fight much."

Faced with mounting criticism, Massoud Barzani, president of Iraq's Kurdish region, made a rare public pronouncement. "There must be no room for negligence and shortcomings," Barzani told the Yazidi community leader, Baba Sheikh. "Measures will be taken against those who were remiss in defending and protecting people."

Meanwhile, the attack on Sinjar has prompted Iraqi and Syrian Kurds to join forces to battle the Islamic State. Some members of the Peshmerga force and hundreds of Yazidi civilians took refuge in areas controlled by Syrian Kurdish fighters, known as the Peoples Protection Units (YPG).

On Tuesday, heavy clashes continued between Kurdish troops and Islamic State fighters in several areas of Nineveh, including around Sinjar, Zummar, Hamdaniya and Makhmour.

"Around 85 percent of the Yazidis live in Sinjar," said Mamo Selim al-Bagsri, head of the municipality in Skhean. He warned that large-scale killing of Yazdis would have an irreversible impact on the community's future. "The only one who is not against Yazidis is God."