Communal tension broke out in Kiphire district of Nagaland on Sunday after a couple travelling in a car was shot dead by two motorbike-riding assailants.Section 144 of the Code of Criminal Procedure that prohibits unlawful assembly has been imposed in the district.According to police, the husband, a government school teacher belonging to the Sangtam tribe was travelling in a car to a village with his wife, a nurse belonging to the Tikhir tribe.Both tribes have an age-old rivalry over land issues."Both the deceased were posted in Kiphire. They went to meet relatives in Shamator village when they were shot dead by two unknown people on a motorbike," a police official told IANS.As per a preliminary report, the two attackers belonged to the Yimchunger tribe, which is also involved in land disputes with various other Naga tribes including Sangtam, Tikhir and Chang, police said.