By Subir Bhaumik
BBC correspondent, Calcutta
The Indian army says it has started a massive offensive against rebels in the north-eastern state of Manipur.
The UNLF rebels have rejected talks with Delhi
They say hundreds of soldiers are closing in on some camps of the United National Liberation Front (UNLF), the largest separatist group in Manipur.
The UNLF is part of a rebel coalition, the Manipur Peoples Liberation Front.
Indian security forces are battling more than a dozen rebel groups fighting Indian rule across the north-east.
Military officials told the BBC the troops were "specifically concentrating" on the UNLF bases in Manipur's western town of Jiribam.
"The rebels have already withdrawn from the smaller bases that ring its main base in the area and have taken some casualties," said military spokesman in Manipur, Colonel Shantanu Dev Goswami.
But he said details were yet to come from the remote area.
The UNLF spokesman, Tombi Singh, told the BBC they have inflicted some casualties on the army columns but admitted their fighters had to "withdraw from some smaller bases."
The Indian army recently signed a series of "agreements" with seven smaller militant groups of the Kuki and Zomi tribes, that guarantees suspension of operations against those groups.
Rebel spokesman, Tombi Singh accused the Indian army of using these groups against the UNLF and its allies.
Earlier this year, the Indian army attacked Manipuri rebel bases, mostly of the UNLF, in the Sajit Tampak and Churachandpur area on the state's borders with Burma. Fighting went on for more than six months.
Though military officials claimed much success, the rebels say they managed to beat back the offensive.
The UNLF also recently rejected talks with the Indian government and called upon Delhi to organise a plebiscite to ascertain whether Manipuris wanted independence or wished to stay on in India.
Delhi rejected the call.