It was earlier reported that 11 people had been killed and four injured in the crash.
The accident occurred when a Kamaz truck drove into the oncoming lane and collided head-on with a Neoplan bus near the town of Petushki at approximately 1.20 a.m. Moscow time [22:20 GMT Wednesday]. The bus subsequently burst into flames.
"Investigators believe that the Kamaz driver fell asleep at the wheel," a traffic police source said. "The section of road where the accident occurred is straight and properly illuminated and the surface of the road was dry at the moment of the tragedy."
The bus, travelling from the Volga city of Ulyanovsk to Moscow, is thought to have had 13 passengers and two drivers onboard.
"All the dead in the accident are residents of Ulyanovsk. According to preliminary information, one of them is a woman. We have no information that there were children among the dead," the source said. He added however that he did not have a complete list of the dead.
He also said that the four injured people had been rushed to hospital in a serious condition. Investigators are working at the scene.