An explosion on a bus killed two people and wounded four others in southern Russia on Sunday, officials said.
The blast occurred when a bus traveling from Pyatigorsk to Stavropol stopped at a bus station in the city of Nevinnomysk, about 1,050 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.
The incident took place in Russia's troubled North Caucasus, which is plagued by violence from feuding criminal groups, remnants of Chechen separatist groups, and other militant groups that target government and police.
It was not immediately clear what caused the blast. An explosion on a bus in the same region on Nov. 22 killed five people and wounded a dozen others in what officials said was a terror attack.
Russia's southern provinces have been plagued by frequent explosions and other violence, some attributed to the local criminal clans and some spilling over from Chechnya, where Russian forces and separatists have fought two wars since 1994.