A million people in northeastern India face famine after rats destroyed most of the rice crop in their state, the International Rice Research Institute has said.

The 2007 infestation spread over to the border areas of Bangladesh and Myanmar in early 2008, "increasing fears of widespread food shortages," the Philippines-based institute said.

It left the Indian state of Mizoram, home to about a million people, with just one-fifth of its monthly rice requirement.

"Aid agencies have reported that many people have been forced onto a diet of wild roots, yam and sweet potatoes," the institute said in its quarterly magazine Rice Today.

IRRI said the rat population boomed after the flowering of a native species of bamboo, an event that occurs only once every 50 years.

"After exhausting the feast of bamboo seeds, the rats turned to the rice crop," it said.

It gave no immediate estimates of the damage on the neighbouring countries.

Rice prices have soared this year, triggering civil unrest in at least two dozen countries according to UN agencies.

India is the world's second largest rice producer after China and is also one of the world's key exporters.