At least 15 tribal people in India's remote northeastern Manipur state have been killed this week in clashes with separatist rebels, police said on Thursday, sparking fears of wider ethnic conflict in the region.

Separatist rebels in neighbouring Nagaland state are campaigning for autonomy in a region that includes parts of Manipur, home to most Kuki tribal people, whom the rebels have attacked in the past.

Before a 1997 ceasefire between the rebels and the central government, the rebels killed nearly a thousand Kukis and burnt down 300 villages in Manipur.

India's northeast is home to a complex web of tribal groups, many of which have launched insurgencies, accusing New Delhi of plundering resources and doing little to improve their lives.

The Kuki Liberation Army (KLA), which wants more autonomy for the Kukis, threatened reprisals on Thursday against the National Socialist Council of Nagaland (Issac-Muivah), or NSCN-IM, if it failed to compensate for killing Kukis.

"If our conditions are not followed by the NSCN-IM we will adopt the policy of an eye for an eye," warned Malsawm Kuki, the information secretary of the KLA.

The NSCN-IM is pushing for self-rule in a "Greater Nagaland", which would include several parts of neighbouring Manipur, Assam and Arunachal Pradesh states where Nagas live in large numbers.

The truce between the NSCN-IM and New Delhi covers its activities in Nagaland state, but peace talks have failed to find a solution to their revolt, which began in 1947 and in which 20,000 people have been killed.

"Tension is very high among the Kukis after these killings. They are angry and we are trying to calm them down," Satkhokai Chongloi, a senior leader of the Kuki Movement for Human Rights, told Reuters from Imphal, Manipur's capital, by telephone.

Authorities have stepped up security measures in Manipur's tribal areas to prevent ethnic violence.

Four districts of Manipur and parts of other northeastern states are home to both ethnic Nagas and Kukis, who have been dragged into a turf war.