A fighter of the Islamic State of Iraq and the Levant (ISIL) holds an ISIL flag and a weapon on a street in the city of Mosul, June 23, 2014.

Fighters from the Islamic State of Iraq and Syria (ISIS) took over Iraq’s biggest dam unopposed by Kurdish fighters, who also lost three towns and an oilfield on Sunday to the Sunni militant group, witnesses said, according to Reuters news agency.

Control of the dam could give ISIS, which has threatened to march on Baghdad, the ability to flood major cities.

Meanwhile, ISIS also seized two small towns in northern Iraq after driving out Kurdish security forces, officials and residents said, according to the Associated Press.

The fresh gains by the Sunni extremist militants have forced dozens of residents to flee from the religiously mixed towns of Zumar and Sinjar, near the militant-held city of Mosul, to the northern self-ruled Kurdish region.

Oil field

Earlier on Sunday, ISIS militants have successfully captured an oil field close to the Iraqi town of Zumar after fighting with Kurdish forces who had control of the area.

ISIS, which had a lightning advance through northern Iraq in June, warned residents in nearby villages along the border with Syria to leave their homes, suggesting they were planning an assault, witnesses said.

ISIS fighters killed 16 Kurdish troops in attacks in northern Iraq, while 30 pro-government forces died battling the jihadists on other frontlines, officials said Saturday.

Zumar is a small Kurdish-majority outpost northwest of Mosul, which used to be under federal government control but was taken over by the Peshmerga in June.

ISIS fighters, who had already been running large swathes of neighboring Syria, launched a blistering offensive on June 9 that saw them capture Mosul, Iraq’s second city, and move into much of the country’s Sunni heartland.

Many government forces retreated in the face of the onslaught, and Peshmerga troops seized the opportunity to fill the vacuum and seize long-coveted areas the Kurds were in dispute with Baghdad over.

 
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