Islamic State fighters seized three small towns in northern Iraq, as well as an oil field and the country's largest dam Sunday, according to Reuters.

The latest victory has been the largest to date and gives the Sunni group the opportunity to topple the Shi'ite-led government.

The long-standing Sunni-Shia conflict has resurfaced since U.S. troops left Iraq, which had been in conflict before the war because the Shia-majority country was ruled by a the minority Sunni leader Saddam Hussein, according to the Council on Foreign Relations.

The difference between the two sects of Islam include times of prayer and interpretations of the holy book The Quran but not much else, according to NBC.

The recent capture of the three towns has caused mass displacement, and the U.N. is calling it a humanitarian tragedy, according to Agence France-Presse.

"A humanitarian tragedy is unfolding in Sinjar," top U.N. envoy in Iraq Nickolay Mladenov told AFP.

Of the estimated 300,000 residents of Sinjar, at least 200,000 have fled into neighboring mountains. A 50-year-old merchant fled with his wife and seven children. He told AFP, "Sinjar has emptied, there are not many people left apart from the 10,000 Sunnis there. The world and the Iraqi government have to do something because some people -- including Yazidis and Christians -- have fled on foot and are now probably stuck in very dangerous areas."

The recent victory for the IS, who are raising their black flags in Syria and Iraq, is another advancement in their battle to restructure the map of the Middle East.

"The (Kurdish) peshmerga have withdrawn from Sinjar, Daash (former Arabic acronym for the IS) has entered the city," Kurdish official Kheiri Sinjari told AFP. "They have raised their flag above government buildings."

The group posted pictures on the Internet of its presence in Sinjar, including its forces patrolling the main street.