The incident took place on saturday night when security forces shot dead civilians believed to be insurgents in Nagaland state.
Of these, up to 11 civilians were killed, according to the PTI agency.
Mon district is located in the northeastern indian state of Nagaland.
Security forces were engaged in an anti-terror operation on saturday night in the Odinga area here.
It is noteworthy that the NSCN (K) and Alpha rebel groups are strongholds in the Mon district.
Nagaland People's Front leader and former prime minister DR Zhejiang says 13 people have been killed.
According to a PDI report, a police officer opened fire on a civilian vehicle as the indian Army conducted a search operation based on information that the Young Ang faction of the NSCN (K) was on the move.
State chief minister Nebu Rio has strongly condemned the incident and ordered a high-level special investigation team to investigate.
He appealed to all parties to keep the peace.
The paramilitary assam Rifles reported that security forces were also killed in the incident, according to the ANI news agency.
The PTI has reported that the indian Army has regretted that this was an unfortunate incident.
Several security personnel were also reported injured. indian Home minister Amit Shah has expressed his condolences to the families of the victims at the event.
He said a high-level inquiry commission set up by the state government would conduct a full inquiry and bring justice to the families of the victims.
'Communication freeze'
Speaking to the BBC, Kekangsim Yimunger, head of the east Nagaland People's Movement, said, "We have received reports that up to 14 civilians have been killed so far.
Many more have been injured and many more are reported missing.
There was no communication at the scene. He also says that SMS and internet service has been discontinued.
