India has opened its longest road-rail bridge in the northeastern state of Assam to boost defences on its sensitive border with China.

Indian Prime Minister Narendra Modi drove an inaugural cavalcade along the 4.9km Bogibeel bridge over Brahmaputra river.

The bridge, near the city of Dibrugarh, will enable the military to speed up dispatch of forces to neighbouring Arunachal Pradesh near China.

India lost territory in Arunachal Pradesh in a 1962 war with China.

The bridge has been designed to bear the weight of India's heaviest 60-tonne battle tanks.