As many as 200,000 people in the Iraqi town of Sinjar have had to flee an invasion by Islamic State militants. The death toll is not confirmed but mass executions often follow such takeovers.

The Islamic State seizure marks the second day of their advance on Kurdish peshmerga forces, the first time militants have done so since declaring their caliphate in June. Also Sunday, Reuters reported the Sunni militant group took over Ain Zalah oilfied and the town of Zumar.

"The peshmerga has withdrawn from Sinjar, Daash has entered the city," Kurdish official Kheiri Sinjari told Agence-France Press, using the former Arabic acronym for the Islamic State.

A Reuters report militants had taken the Mosul Dam has not been confirmed although the peshmerga has confirmed clashes are taking place. If militants take over the damn, it would give them the ability to flood major Iraqi cities.

Top U.N. envoy in Iraq Nickolay Mladenov called the situation in Sinjar a “humanitarian tragedy.”

"The United Nations has grave concerns for the physical safety of these civilians," he said in a statement Sunday.

"Hundreds fled leaving vehicles and a huge number of weapons and munitions and the brothers control many areas," the Islamic State said in a statement on its website. "The fighters arrived in the border triangle between Iraq, Syria and Turkey." 

A senior source in the peshmerga told AFP troops "have withdrawn to mountain areas and are getting reinforcements.” The U.N. is particularly concerned for those who have fled to the mountainous region, as it is surrounded by jihadi forces.

Sinjar has been under threat of attack from militants for several weeks and peshmerga forces have been requesting weapons and aid from the United States to combat jihadi advances. Unconfirmed reports indicate militia worked with the peshmerga to combat militants in Sinjar. In Syria, they had joined forces briefly with the Free Syrian Army to do just that.

In Iraq, Shiites have been the U.S.-imposed ruling sect since Nouri al-Maliki, the former Shiite dissident under the rule of Baathist Saddam Hussein, was elected prime minister. Sunni militant group ISIS took over Mosul, Iraq’s second largest city in June. It has been threatening to take over the Baghdad and push the country into a sectarian civil war akin to what plagued the country in 2007.