Nagaland remained on the boil on Monday as an apex tribal body claimed the death toll in firing by security forces on civilians in Mon district mounted to 17, even as police maintained that 14 people were killed.

The first firing incident in which six civilians were killed, occurred when army personnel mistook coal mine workers returning home in a pick-up van on Saturday evening, to be insurgents belonging to the Yung Aung faction of the banned outfit NSCN (K).

As workers failed to reach their homes, local youth and villagers went in search of them and surrounded army vehicles. In the ensuing melee, one soldier was killed and many of the vehicles burnt down. Security forces who fired in self-defence killed another nine civilians, members of the tribal body Konyak Union claimed.

Police officers, however, said seven civilians were killed.

Rioting spilled over into Sunday afternoon when an angry mob vandalised the offices of the union and an Assam Rifles camp in the area, setting fire to parts of it.

At least two more persons were killed, as security forces fired back at attackers, according to the union members, but the police maintained that one person died in the incident.

In a sudden move, tribal bodies, civil societies and student bodies Monday imposed shutdowns of different durations ranging from six to 12 hours across the state.

The influential Naga Students' Federation (NSF) has announced five days of mourning, while also asking tribals not to participate in any celebration during the period.

Of the injured, the condition of six are stated to be critical and they are undergoing treatment at hospital, officials said.

Nagaland Chief Minister Neiphiu Rio and Deputy Chief Minister Y Patton, who were in Delhi to meet Union Home Minister Amit Shah, rushed back on Sunday and are now headed for Mon to take stock of the situation.