The UN toll includes members of the army, police engaged in combat, the Kurdish peshmerga, interior ministry forces and pro-government paramilitaries.
The UN statement also said at least 926 civilians were killed, bringing to 2,885 the number of Iraqis killed in acts of terrorism, violence and armed conflict last month.
The spike in casualties comes as a major offensive to retake the IS stronghold of Mosul, Iraq's largest military operation in years, enters its seventh week.
Kubis said the growing death toll was largely a result of the jihadists' ferocious defence of Mosul, the city where they proclaimed their now crumbling "caliphate" in 2014.
"Daesh (IS) has been employing the most vicious tactics, using civilian homes as firing positions as well as abducting and forcibly moving civilians, effectively using them as human shields," he said.
The US-led coalition assisting anti-IS forces in Iraq and Syria admitted Thursday to "inadvertently" killing 54 civilians in both countries between March and October.
"Although the coalition makes extraordinary efforts to strike military targets in a manner that minimises the risk of civilian casualties, in some cases casualties are unavoidable," the coalition said in a statement.
A July 18 strike that killed 100 IS fighters also killed as many as 24 civilians, the statement added.