By Subir Bhaumik BBC News, Calcutta

Indian forces have been fighting Assam rebels for two decades

ULFA sources say that a letter from the prime minister's office has been sent to the group's chairman saying that any issue can be raised in the talks.

The sources say that includes demands for Assam's sovereignty.

The rebel group has so far refused to join talks because of disagreements with Delhi over the sovereignty issue.

'Face-saving formula'

Leading Assamese writer Indira Goswami - who is mediating between the ULFA and the central government - told journalists in Assam's commercial capital, Guwahati, that the letter had been handed over.

Ms Goswami said it had been written by India's National Security Adviser, MK Narayanan, and had been delivered to the ULFA chairman Arabinda Rajkhowa.

She said the ULFA should now agree to start negotiations, since Mr Narayanan has clearly said in the letter that the rebel leaders could raise the issue of Assam's sovereignty.

She said Delhi has now "set the stage", and that the ULFA should use the opportunity without any delay.

So far there has been no official reaction from the ULFA.

But the violence seen in recent days - including the blowing up of bridges and pipelines - has stopped since Saturday morning.

Analysts say the government has actually offered the ULFA a face-saving formula by which it can raise the sovereignty issue with no real promise that the Indian government would ever seriously discuss the issue.

Most rebel groups in Assam, including the National Democratic Front of Bodoland (NDFB) have started negotiations with Delhi.