Tribal guerrillas firing rocket-propelled grenades and detonating improvised explosive devices have killed 20 Indian soldiers in the northeastern state of Manipur, according to officials.
N Herojit, superintendent of police of Manipur's Chandel district, where the attack occurred, said the ambush happened at 7am local time on Thursday.
Another 11 soldiers were 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.
Manipur, with a population of 2.5 million, has struggled for years with an armed insurgency in which several tribal armed groups are active.
However, it was not immediately clear which group carried out the latest ambush.
"We do not know as yet who is responsible," said a spokesperson at army headquarters in the capital, New Delhi, adding that details were still coming in from the scene.
Narendra Modi, Indian prime minister, condemned the attacks in a Twitter post on late on Thursday saying: "Today's mindless attack in Manipur is very distressing. I bow to each and every soldier who has sacrificed his life for the Nation."
Security analysts suspect the attack was in retaliation for the reported killing of a woman by soldiers on Monday.
The district observed a complete shutdown on Wednesday in protest at the killing.
In April, fighters armed with automatic weapons fired at two lorries carrying Indian paramilitary soldiers in neighbouring Nagaland state, killing eight of them.
Security forces have been given sweeping shoot-to-kill powers in "disturbed areas" under India's 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.
Legislators recently lifted the act in one state, Tripura, that borders Bangladesh.
Indian forces in the mountainous northeastern region, known for its natural beauty, have been battling armed groups for decades.
Most of the main armed groups in Manipur are not engaged in ceasefire talks with the Indian government, unlike those in other remote northeastern states.
The groups accuse the Indian government of exploiting the region's rich natural resources while neglecting local development.