Government officials in Assam's state capital Dispur believe the insurgent group Isak-Muivah -- a faction of the militant Nationalist Socialist Council of Nagaland -- was involved in the brutal killing of the nine Karbi tribe youths.
The Karbi tribe lives mainly in the Karbi Anglong hills bordering Nagaland state.
The partially decomposed bodies of the youths were found Friday in a public drain near Dimapur, the largest city in Nagaland, the Times of India reported.
Assam police added extra patrols and checkpoints in the Karbi Anglong region Monday as a precaution against further violence.
"The situation is still tense," Assam state Home Commissioner G.D. Tripathi told the Times.
"We have asked for 25 companies [of security forces] and 10 of them have already been deployed in the district. Our prime objective at the moment is to contain the situation," Tripathi said.
The Nagaland state government has set up a special team to investigate the deaths of the youths, whose ages weren't given.
The Times reported an unnamed source said the killings were done by the Isak-Muivah faction, the same group that was engaged in a gun battle with the Karbi Peoples Liberation Tigers Dec. 27 in Assam's Karbi Anglong region.
Isak-Muivah has been negotiating with the Indian government for 15 years for the creation of a greater Naga state. The new state would move Naga-dominated areas of Assam, Manipur and Arunachal Pradesh states into the present state of Nagaland -- created in 1963 to appease Naga tribal wishes for a homeland.
Governments and political parties in Assam, Manipur and Arunachal Pradesh oppose the rebel demand.
Violence stemming from competing territorial and independence claims by the two tribes has led to more than 3,000 Karbi and Rengma Naga people in Assam fleeing their homes, the BBC reported.
Assam police official M.J. Mahanta told the BBC that more than 1,600 Rengma Nagas and about 1,500 Karbis are living in makeshift camps, mostly set up in local schools in Assam's Karbi Anglong district.
"The displaced villagers are suffering from lack of essentials like food and clothes and living in the bitter cold," Sushanta Roy, a newspaper editor in Karbi Anglong's main town of Diphu, told the BBC.
In the Dec. 27 gun battle, rebels belonging to the Karbi People's Liberation Tigers attacked Rengma villages in the Karbi Anglong district and killed six people, five of them women.
The BBC reported the rival so-called Rengma Naga Hill Protection Force retaliated by killing three members of the Tigers, a group that wants a separate homeland for the Karbi tribe.
Tribal attacks are common in India's remote and restive northeast area that for years was off limits to tourists.
Suspected insurgents opened fire on migrants playing cards in an Assam hill village in early November, killing seven people and wounding 10, the Press Trust of India reported at the time.
The village is in Assam's Goalpara district near the border with Meghalaya state.
The attack, blamed on members of the Garo National Liberation Army based in Meghalaya, targeted migrant workers as they played cards, the report said.
The Garo rebel group has been fighting for a separate state since 2009 and has been blamed for several killings, kidnappings and extortion in the region.