State police in Nagaland have filed an official complaint against 21 Para Special Forces of the Indian Army in connection with a counter-insurgency operation that led to the death of 14 villagers.
The state police have accused the Indian army of killing more than a dozen locals.
"The 21 Para Special forces of the Army opened fire resulting in the death of many inhabitants of the village Oting in Mon district of Nagaland near the border with Assam, the state police has alleged in its First Information Report (FIR) against the Army unit. It further alleges that security forces intended to murder and injure civilians," Indian News Channel NDTV reported.
As the situation remains tense, gatherings of four or more people are banned in Mon District.
Meanwhile, the Indian Army on Sunday expressed regret over the incident, saying that the security forces also suffered severe injuries and one soldier died from his wounds.
According to reports, soldiers ambushed a vehicle on Saturday evening, killing six labourers who were returning from a coal mine to their village. Media reports suggest officials in the security establishment said the incident was the result of an Army operation in which the victims may have been mistaken for insurgents from the Yung Aung faction of the banned National Socialist Council of Nagaland (NSCN), a Naga secessionist group.
Later, angry villagers set the army vehicle on fire and attacked army camps, after which seven more people, including six civilians and one soldier, were killed in the clashes.
Nagaland state chief Neiphiu Rio described it as "highly condemnable" and said a "high-level" special investigation team would investigate the case to ensure "justice is delivered". Meanwhile, he has called for peace to be maintained.
Federal Home Minister Amit Shah has also his sorrow over the deaths, saying he was "anguished". He also talked of a probe to be conducted by the Nagaland state government.
Meanwhile, the opposition parties have demanded a detailed inquiry.
Congress Leader Rahul Gandhi has sought a response from the federal government about the deaths and called the incident "heart-wrenching".
A five-member delegation of the Trinamool Congress (TMC) party is set to visit Nagaland state on Monday to meet the families of those who died in the incident in the Mon District.
What's Happening In Nagaland?
Nagaland has witnessed decades of armed rebellion against Indian government rule. However, in recent years, the region has been relatively quiet as armed groups started negotiations with the Indian government.
Last week, Nagaland state chief Neiphiu Rio visited Delhi and met Home Minister Shah to discuss the state's political situation.
"We have been waiting for the past 24 years for the Indian government to resolve the Naga political issues. However, security forces created havoc just when the solution was in sight," Chuba Ozukum, adviser to the Naga people's tribal body Naga Hogo, told Indian daily The Economic Times.
After the attack, demands for the repeal of the Armed Forces Special Powers Act (AFSPA) - a law that same say grants impunity to security forces operating in the area - have been more urgent.
Nagaland has been declared a "disturbed area" by the Union Home Ministry under AFSPA, which empowers security forces to conduct operations anywhere and arrest anyone without prior notice.
AFSPA has been part of the north-eastern states since 1958. Nagaland was declared a state in 1963.
The Nationalist Democratic Progressive Party, a regional political party in Nagaland, issued a statement saying that the state government should take up the cause of repealing AFSPA.