At least 14 civilians and one member of the security forces have died after Indian forces mistook a group of villagers for insurgents and opened fire.
A local community leader, Nyamtow Konyak, said the civilians killed were coal miners.
On Saturday, an investigation into the killings was launched by Nagaland state’s top official, Neiphiu Rio.
In a tweet, he condemned the actions of the troops and passed on his condolences to bereaved families.
An army officer, speaking anonymously, shared that the ambush had been planned for a week as intelligence indicated insurgents were planning to attack soldiers in the area, 50 miles east of Gauhati, capital of Assam state.
It happened in a remote area in the northeast of the country, along the border with Myanmar.
Six people were killed as the soldiers mistook them for militants, opening fire at a lorry.
Angry villagers responded and burned two army vehicles. The troops then fired at them, killing a further eight people, according to Reuters.
One soldier also died during the clash.
Violence erupted again on Sunday, as nearly 200 villagers attacked the army camp in Mon district.
The residents set fire to the army quarters and rampaged the camp.
Police and local student leader, Yuwong Konyaki, said the army retaliated by firing live ammunition into the crowd, killing a further two people.
Police reinforcements rushed to the area to quash the violence.
Amrit Shah, India’s home minister, said the government would investigate the killings and shared his remorse over the “unfortunate incident”.
In a statement, the Indian army said it “deeply regretted” the incident and its aftermath.
“Security forces have suffered severe injuries in the incident, including one soldier who succumbed to the injuries,” it said.
“The cause of the unfortunate loss of lives is being investigated at the highest level and appropriate action will be taken as per the course of law.”
The army said it had received “credible intelligence” that a “specific operation was planned” by insurgents in Mon district.
The government continues to battle dozens of insurgent groups in India’s northeast region.
Their demands range from independent homelands to maximum autonomy within India.