Indian soldiers and paramilitary troops have been on patrol to try to quell the unrest that started on Friday, triggered by long-standing territorial disputes.

"Incidents of arson and violence were reported overnight from several places with the death toll now put at 22," Hagrama Mohilary, head of the Bodoland Territorial Council, a local government body, told AFP by telephone.

"The situation is tense and volatile and we want more security forces, especially reinforcements of army soldiers."

Local television channels broadcast pictures of several homes that had been set ablaze by rioters.

"Police, army and paramilitary troopers have intensified patrols and a 24-hour, indefinite curfew has been imposed," Assam Forest Minister Rockybul Hussain said from the worst-hit district of Kokrajhar.

Police issued shoot-on-sight orders late on Monday after rioters burnt shops and houses and attacked rival gangs. The orders mean that mobs breaking the curfew could be shot without warning.

Authorities said an estimated 40,000 people -- many of them women and children -- had fled their homes to take shelter in relief centres set up at designated government facilities.

"We are providing food and basic medicines to those in need," Nazrul Islam, another senior state minister, told AFP by telephone.

The Press Trust of India news agency said the fighting started when two Muslim student leaders were shot and seriously injured in Kokrajhar, leading to revenge strikes on Bodo groups.

Northeast India, which is linked to the rest of the country by a narrow land bridge, has seen decades of friction among ethnic and separatist groups, though some of the biggest rebel movements have recently started peace talks with the government.