Civilians may have also been wounded in the blast, it said.
A vehicle rigged with a bomb exploded next to the bus -- which was transporting off-duty soldiers, according to the country's Anadolu state news agency. The blast happened near Erciyes University in the central province of Kayseri, according to a tweet from Anadolu.
Turkey's President Recep Tayyip Erdogan and Prime Minister Binali Yıldırım are being briefed on the situation, according to the news agency.
A video showed a decimated bus with smoke rising out of the vehicle.