Separatist groups in the region have stepped up violence ahead of independence day celebrations on Friday.

Many rebels want a boycott of festivities marking the date India gained independence from Britain in 1947.

In a separate incident, security forces say they have killed at least six rebels in north-eastern Assam state.

Security precautions

Six people were killed and 12 injured in the Manipur attack.

Rebels blew up a bridge in Lilong, near the state capital, Imphal, as a bus was crossing it, police said.

Indian troops face more than a dozen rebel groups in Manipur

Police suspect the involvement of the Manipur People's Liberation Front.

In Tripura, rebels belonging to the All Tripura Tiger Force attacked a Bengali settlement at Lembucherra near the state capital, Agartala, killing two people and injuring six others, police said.

The Indian authorities have tightened security in the north-east ahead of independence day.

Indian Railways have suspended night services until Saturday, and federal troops have been deployed to guard vulnerable areas.