The United Nations said the capture of the Iraqi town of Sinjar by jihadist fighters has sent up to 200,000 people into flight and sparked a "humanitarian tragedy" amid fears for their lives.

"A humanitarian tragedy is unfolding in Sinjar," top UN envoy in Iraq Nickolay Mladenov said in a statement.

He was speaking after Islamic State captured the northern town, which lies near the Syrian border, and was already home to thousands of previously displaced families.

The UN statement said some reports put the number of people forced to flee by the IS takeover at 200,000.

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

It said it was particularly alarmed by the fate of those civilians who fled in the Jabal Sinjar mountains and who could be trapped in an area completely surrounded by jihadist fighters.

"The humanitarian situation of these civilians is reported as dire, and they are in urgent need of basic items including food, water and medicine," the statement said.

Sinjar was controlled by Kurdish troops but they withdrew on Sunday, the second consecutive day of losses for the peshmerga, who also relinquished the town of Zumar and two nearby oil fields yesterday.

Sinjar is a historical home for the Yazidis, a Kurdish-speaking minority which follows a pre-Islamic faith derived in part from Zoroastrianism.

They have been targeted by the jihadists, who call them devil worshippers.

Mr Mladenov urged authorities of the autonomous Kurdish region in northern Iraq and the federal government in Baghdad to cooperate fully in addressing the crisis.