MOSCOW (REUTERS) - An explosion ripped through a bus in the southern Russian city of Volgograd on Monday, killing at least five people and injuring 12 others, an Emergency Situations Ministry official said.

There were conflicting reports about the cause of the blast.

Interfax news agency cited an unnamed source as saying it was a bomb, while RIA cited city police as saying preliminary information indicated that the explosion was an accident caused by a problem with gas equipment.

Volgograd is a large city that lies north of the North Caucasus, a string of mostly Muslim provinces where the government is trying to quell an Islamist insurgency rooted in two post-Soviet separatist wars in Chechnya.