An explosion on a bus Sunday killed two people and wounded four others in southern Russia, officials said. The blast occurred when a bus traveling from Pyatigorsk to Stavropol stopped at a bus station in the city of Nevinnomysk, about 1,700 kilometers (1,050 miles) south of Moscow, said Sergei Kozhemyaka, a spokesman for the Emergency Situations Ministry's branch in southern Russia. The explosion in the back of the bus killed two passengers and wounded another four, including one who was in serious condition, Kozhemyaka said. It was not immediately clear what caused the blast. An explosion on a bus in the same region on November 22 killed five people and wounded a dozen others in what officials said was a terror attack. Russia's southern provinces have remained restive, plagued by frequent explosions and other violence, some of which was spilling from Chechnya and other attributed to local criminal clans.