Heavily armed separatist rebels have ambushed a paramilitary convoy in India's remote northeast, killing eight soldiers and injuring six, officials said Monday.

The militants opened fire on the troops before exploding a small bomb during the attack on Sunday in underdeveloped Nagaland close to the border with Myanmar, the state's police chief said.

"Militants first resorted to firing and then triggered an improvised explosive device, killing eight troopers on the spot and seriously injuring six more," Nagaland police chief L.L. Doungel told AFP by phone.

Another four Assam Rifles troopers were missing, with a search underway in the area, about 200 kilometres (124 miles) north of the state capital Kohima, an official said on condition of anonymity.

Police suspect the attack was carried out by the outlawed National Socialist Council of Nagaland, which has been campaigning for decades for a homeland for indigenous Naga tribes to be carved out of India's northeast states.

Last month a faction of the council called off a longstanding ceasefire, saying New Delhi was not serious about meeting the movement's demands.

The troopers in two vehicles came under attack as they returned to their camp from collecting drinking water nearby, the police chief said.

"There was heavy firing with the troopers retaliating but it was not immediately known if there were any casualties on the militants' side," Doungel said.

"A massive operation has since been launched in the thickly forested area," he said.

India's seven northeastern states have long been plagued by insurgent violence. They are home to dozens of tribal groups and small guerrilla armies that resist rule from New Delhi and often compete against each other.