Rescuers are having to scour through the rubble by hand
The governor of Tal Afar told the BBC the current toll of 250 dead and 350 injured was expected to rise sharply.
Poor families in the Yazidi community hit by the bombings often crowd as many as 30 people in one home, he said.
The bombing of two Yazidi villages near Mosul was one of the worst attacks in more than four years of war in Iraq.
It already surpasses the killing of just over 200 people in car bombings and mortar fire in Baghdad's Shia stronghold of Sadr City last November.
The BBC's Mike Wooldridge, in Baghdad, says other local officials have also indicated the death toll could quickly rise.
In the latest violence, a bomb attack in a central square in the capital was reported to have killed four people.