By Subir Bhaumik BBC correspondent in Calcutta

Barua (right) said Ulfa would not tolerate Hindi "oppression"

The United Liberation Front of Assam said it would drive out Hindi speakers if they did not leave the mainly Assamese-speaking state immediately.

The threats came after mobs in the nearby state of Bihar attacked Assam passengers on five trains on this week.

The mobs were angry at alleged job bias against Biharis in Assam.

The chief of Ulfa's military wing, Paresh Barua, told the BBC in an interview on Friday that his organisation was planning large-scale retaliation for the train attacks.

In response, the Assam Government vowed to protect all Indian citizens living in the state.

Assam's police intelligence chief Khagen Sarmah told the BBC that all police stations had been put on high alert.

"Ulfa is trying to exploit Assamese sentiment to regain its lost popularity. It may strike at soft targets like Bihari workers at remote construction sites and coal mines in northern Assam. We are making all possible arrangements," Mr Sarmah said.

Cinemas warned

The rebel threat came after at least 50 train passengers were injured in attacks by armed mobs in Bihar on earlier this week.

The youths were protesting over alleged discrimination against Biharis who had tried for jobs with Indian Railways in Assam.

Assamese train passengers were targeted by the Bihar mobs

The attacks happened at five stations where trains bound for Assam had stopped.

The mobs singled out passengers from Assam and other states in India's north-east, and officials said even women and children were beaten up.

Mr Barua told the BBC that Hindi-speakers in Assam would face attacks if they did not leave.

"We will not tolerate the aggression of the Hindi-speaking peoples; we will drive them out of Assam and the rest of north-east India," Mr Barua said.

He also warned cinemas not to screen Hindi films after Friday.

Analysts say Ulfa has been pushed into a corner by Indian military operations in Assam and increasing pressure on it to vacate its safe bases in southern Bhutan.

They say Ulfa may try to exploit anti-Hindi sentiment as it did in November and December 2000, when more than 100 Hindi-speaking people from Bihar and Rajasthan were killed by Ulfa.