Mosul, Iraq's second city, was retaken from IS in July after a massive months-long offensive.
"Waad Allah forces are repelling an IS attack southwest of Mosul in the Hatra desert" some 100 km southwest of Mosul, said a spokesman for the Hashed unit.
The Hashed is an umbrella group of paramilitary auxiliaries formed in 2014 to support Iraqi regular forces after IS swept across swathes of northern Iraq.
In a series of online posts, Waad Allah said IS sent "numerous suicide bombers" to attack its forces, and gave a death toll of 24 jihadists and four of its own men.
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But despite a series of stinging defeats, IS in Iraq retains the ability to launch attacks in areas declared "liberated" months previously.
After losing Mosul in July, IS has also just lost Raqa, its "capital" in Syria.
The Hashed took over the Hatra area in April this year after IS forces were ousted.
The ancient walled city of Hatra in northern Nineveh province is on the UNESCO World Heritage List.