Previous estimates put the total number of dead at around 250. The revised death toll makes the bombings the deadliest such attacks since the start of the war in 2003.
Hundreds of victims have been taken to regional hospitals, which were struggling to treat all the wounded.
Interior Ministry spokesman Brigadier General Abd al-Karim Khalaf said two tons of explosives were used in the blasts, which collapsed an entire neighborhood near the Syrian border, trapping families beneath the buildings.
Most of the victims were Yezidis, a small Kurdish-speaking sect that has been targeted by Muslim extremists who consider its members to be blasphemers.