Indian forces Tuesday hunted down and inflicted "significant casualties" on rebel groups allegedly involved in the killing of 20 soldiers in the remote northeast of the country last week, officials said. The army said it "engaged two separate groups of insurgents" in the mountainous forests close to the border with Myanmar after armed rebels ambushed a military convoy in Manipur state last Thursday, killing 20 and wounding 12 soldiers. It was one of the deadliest attacks in Manipur, which borders Myanmar and has been plagued by insurgent violence for decades. "Significant casualties have been inflicted on them," Ranbir Singh, additional director of general military operations, said at a press conference in New Delhi, without giving further details. The Press Trust of India, however, cited unnamed sources saying Indian forces crossed into Myanmar and killed some 15 militants who escaped through the porous border, after coordination between the two countries. The Indian army launched a massive search involving hundreds soldiers and helicopters last week to track down the attackers. During last week's attack, the rebels had fired rocket-propelled grenades, detonated improvised explosive devices and targeted about four military vehicles that were on a routine patrol in a remote area of Chandel district, about 120 kilometres (75 miles) south of state capital, Imphal. The remote Hindu-majority state of Manipur is home to dozens of tribal groups and small guerrilla armies fighting New Delhi's rule. Their demands range from secession to greater autonomy. At least 50,000 people have lost their lives in insurgency-driven violence in six of India's seven northeastern states since the country's independence from Britain in 1947, authorities say. The seven states are home to dozens of guerilla and tribal groups, which often compete against each other.