GUWAHATI - Unidentified tribal guerrillas, firing rocket-propelled grenades and detonating improvised explosive devices, killed 20 soldiers on Thursday in an ambush in Manipur, an army spokesman said.
Manipur, with a population of 2.5 million, has struggled for years in the grip of an armed insurgency in which several tribal militant groups are active.
Eleven soldiers were also wounded in the most deadly attack in the region in recent years, which targeted a troop convoy heading for Imphal, the capital of the state bordering Myanmar.
"We do not know as yet who is responsible," said a spokesman at army headquarters in New Delhi, adding that details were still coming in from the scene.
Defence Minister Manohar Parrikar condemned the attack and said "those who committed the cowardly act would be brought to book", his spokesman said in a series of tweets.
Security analysts suspect the attack, in the state's Chandel district, was in retaliation for the reported killing of a woman by soldiers on Monday. The district observed a complete shutdown on Wednesday in protest over the killing.
Security forces have struggled to contain unrest in the state, despite being given sweeping shoot-to-kill powers in so-called 'disturbed areas' under the controversial Armed Forces Special Powers Act (AFSPA).
The law operates in most northeastern states, where clashes claimed 450 lives last year, the South Asian Terrorism Portal says. Lawmakers recently lifted the act in one state, Tripura, that borders Bangladesh.