Guwahati: Tribal guerrillas firing rocket-propelled grenades and detonating improvised explosive devices killed 20 soldiers on Thursday in the mountainous Manipur state, an army spokesman said.

Manipur, with a population of 2.5 million, has struggled for years with an armed insurgency in which several tribal militant groups are active. It was not immediately clear which group carried out Thursday’s ambush.

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.

‘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.

Prime Minister Narendra Modi 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, 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 at the killing.

Security forces have been given sweeping shoot-to-kill powers in ‘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.