Myanmarese troops are locked in pitched battles with thousands of Naga guerrillas and have destroyed half a dozen "mobile camps" of the rebels, an insurgent leader said on Friday.
An Indian military commander confirmed that "something serious" was taking place in the thick jungles of Myanmar bordering India but could not provide details.
Kughalo Mulatonu, a leader of one of the factions of the separatist National Socialist Council of Nagaland (NSCN-Khaplang), said heavy fighting was going on between the entrenched Naga guerrillas and Myanmarese forces.
The group, along with the dominant NSCN-IM faction, has signed a ceasefire with New Delhi.
He said at least three Myanmarese brigades (about 10,000 personnel) had launched a full-scale military operation, and warned that the Indian government would be responsible if the group pulled out of its truce.