Kughalo Mulatonu, a leader of the National Socialist Council of Nagaland (NSCN) which is fighting for a tribal homeland in India's northeastern state of Nagaland, said pitched battles had been fought in the past few days.

At least three brigades (about 10,000 personnel) are involved in the operation in military-run Myanmar, he said.

"We have lost at least five to six mobile camps since the offensive began," Mulatonu told AFP by telephone from Kohima, the Nagaland state capital.

"It is a war-like situation with weapons such as sub-machineguns, AK-47s, carbines, and light machine guns used by us and the enemy troops during the continuing gunbattle."

There has been no immediate confirmation of the military offensive from Myanmar although an Indian army commander said intelligence inputs indicated "something serious" was taking place inside the neighbouring country.

"We don't exactly know the details but we can sense something serious happening inside Myanmar and hence a full alert (has been) sounded all along the border to prevent infiltration of rebels," said the commander, who requested anonymity.

The rebel leader said the NSCN has at least 50 camps with some 5,000 guerrilla fighters entrenched in fortified bunkers in the Sagaing Division of Myanmar.

At least four other militant groups from India's northeast, where numerous tribal and ethnic groups are fighting for greater autonomy or independence, have training camps in northern Myanmar's thick jungles -- all of them sheltered there under the patronage of the NSCN.

Mulatonu said that all top leaders, including chairman S.S. Khaplang, are safe.

"We are not going to leave Myanmar and shall fight and repulse the Myanmarese forces. In many places we have planted landmines and timebombs to prevent the army from advancing."

The rebel leader claimed at least 5,000 tribal Naga villagers living on the periphery of their camps have fled their homes since the operation began.

"The Myanmarese troops have now surrounded several villages, with the civilian population fleeing for safety and taking shelter in the adjoining jungles," the NSCN leader said.

The NSCN's Khaplang faction has been observing a ceasefire with New Delhi since 2001 although peace talks are yet to begin.