The Kandilli observatory's estimate is based on the strength of the quake and the structure of the housing in the area.
"We are estimating a death toll between 500 and 1,000," Mustafa Erdik, head of the Kandilli observatory, told a televised news conference Sunday.
The deputy prime minister says around 45 buildings have collapsed in the town of Ercis and the city of Van. Ercis sits on a geological fault line.