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MOSCOW, Dec 9 (Reuters) - A blast on a bus in southern Russia killed two people on Sunday, emergency services said, less than a month after five people died when a nail bomb exploded on a bus in a neighbouring province.

The local prosecutor's office in Stavropol region was treating the blast as a terrorist bomb, RIA Novosti news agency said, quoting a an unnamed official. Other sources said the blast could have been caused by an exploding gas canister.

Stavropol borders Russia's north Caucasus, a volatile region wracked by violence centred on Chechnya, which has fought two separatist wars since 1994.

The bus had been travelling from the city of Stavropol to another town when an explosion started a fire on the bus while it was parked at a station in the town of Nevinnomyssk.

Television pictures showed the burnt out shell of a red and white intercity bus. The back of the bus, above the engine, had been blown apart and its roof was curled back.

The blast killed two women and injured at least four people, news agencies quoted local emergency workers as saying.

On Nov. 22 a bomb tore apart a bus on the border of North Ossetia and Kabardino-Balkaria, an attack blamed on rebels and in October a bomb on a bus during the morning rush hour in the central Russian car making city of Togliatti killed at least eight people. (Writing by James Kilner; Editing by Ralph Boulton)

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