Recapturing Sinjar would effectively cut off the supply line between the IS strongholds of Raqqa and Mosul.
When the town fell to IS last year, tens of thousands of people from the Yazidi religious minority were trapped after fleeing up Mount Sinjar.
Hundreds of men were killed and thousands of Yazidi women and girls captured and used as sex slaves.
The attack on Sinjar was one of the reasons the US began air strikes against IS positions in Iraq in August 2014, amid a warning of genocide.
It expanded its air campaign into Syria the following month. Yazidis, whose religion includes elements of several faiths, are considered infidels by IS.
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