Hundreds of thousands of Iraqi civilians are evacuating towns in northern Iraq, prompting the United Nations to warn of a humanitarian "tragedy."

Thousands are fleeing because advances by the Islamic State of Syria and Iraq, or ISIS, an Islamic, al-Qaida affiliated militant group that has steadily taken over Iraqi towns since June. 

According to The Los Angeles Times, ISIS and allied forces have taken over new areas in the Nineveh plains of northwestern Iraq. They captured the town of Sinjar, which had previously provided shelter for ethnic and religious groups who were trying to evade Sunni violence. 

Around 200,000 civilians, most of whom are from the Yazidi minority who follow a pre-Islamic, Zorastrian-esque religion, have fled to the mountains to escape persecution. 

ISIS views Yazidis and Shia Muslims as "infidels," prompting the two minority groups to evacuate the area out of fear of violence against them. Many Yazidis and Shias have reportedly been killed, and their places of worship have been destroyed. 

Arabic Christians have also fled the area after ISIS captured Mosul in June, which is Iraq's second-biggest city and a longtime center for Christianity in Iraq. 

U.N. official Nickolay Mladenov said Sunday that the situation is a "humanitarian tragedy" and that there is an "urgent need" for water, food and medicine. 

"I call on all Iraqi authorities, civil society and international partners to work with the United Nations to ensure the delivery of life-saving humanitarian assistance," he said, according to the LA Times. 

ISIS pushed out Kurdish peshmerga forces after prompting the Iraqi military to retreat to the south in June, leaving a dangerous security vacuum. 

The gains of the jihadist group following weeks of relative stability raises concerns about the ability of Kurdish forces to hold off the Islamic fighters. 

Sunni Muslims and other groups, who are unhappy with the predominantly Shiite government led by Prime Minister Nouri al-Maliki, have formed alliances with ISIS, which has increased the group's power. 

The Islamic State said it wants to capture Baghdad. However, so far, the capital has remained under the control of the central government and is defended by the Iraqi military. 

Yet, in addition to capturing more towns, ISIS has taken over two oil fields in northwest Iraq and a major dam in the region. The Islamic State is reportedly selling oil that they seized from oil fields in Syria and Iraq on the black market. 

The insurgents have reportedly seized control of the Mosul Dam, which is the biggest electricity-generating dam in Iraq. The capture of the dam gives ISIS the ability to flood Iraqi cities or withhold water from farms. 

Iraqi state television reported Sunday that "terrorist gangs of the Islamic State have taken control of Mosul dam after the withdrawal of Kurdish forces without a fight."

A Kurdish official told Reuters that the dam was still under the control of the Kurdish peshmerga troops, even though he said the dam had been captured by ISIS. 

The withdrawal of the Kurdish troops was a blow to the Iraqi forces who were holding off the Sunni insurgents. 

ISIS views Iraq's majority Shias as heretics and seeks their ultimate destruction. The group, which declared a caliphate in parts of Syria and Iraq to rule over Muslims, is the biggest security threat to Iraq since the fall of Saddam Hussein in 2003.

According to The Guardian, a U.S. state department official said Washington is "actively monitoring" the situation and is working to coordinate an offensive against ISIS through Iraq's military and Kurdish forces. 

ISIS also controls cities in Iraq's Tigris and Euphrates valleys, which are north and west of Baghdad, as well as a region that spans from Syria's Iraqi border to the west in Aleppo. 

Iraq's Kurds expanded areas under their control in recent weeks, but the loss of towns on Sunday in the Kurdish-held regions has weakened claims that ISIS is helping the Kurdish cause.