Ulfa has been fighting Indian forces for two decades

The powerful United Liberation Front of Assam (Ulfa) says it could not accept the offer because of a demand that the group give up violence.

Last week, Indian Prime Minister Manmohan Singh had urged the rebels to hold talks.

But he added that Delhi would only talk to those groups who renounced violence.

Mr Singh's offer was made in a letter written to a leading Assamese writer, Indira Goswami, who has been nominated by the rebels to act as their mediator.

But in a statement released on Friday, Ulfa commander Paresh Barua said the prime minister had reneged on a promise to hold unconditional talks.

"The letter says the Indian prime minister has not put any preconditions. But at the same time the letter says he will hold talks only with those who abjures violence - and that in itself is a precondition."

"Under the circumstances it is simply not possible for Ulfa to proceed further for holding talks," he said.

Appeal

The Indian prime minister had visited Assam while on a tour of the north-east last month, during which he had appealed to rebel groups in the region to begin negotiations with the government.

India's north-east is home to more than 200 ethnic and tribal communities and more than 20 rebel groups fighting for greater degrees of independence or tribal rights.

Ulfa is one of the most powerful groups in the region and has been fighting Indian security forces for more than two decades.

The group has recently indicated that it is willing to begin negotiations as long as they were unconditional.