India said on Friday thousands of illegal immigrants from neighbouring Muslim-majority Bangladesh had produced false documents to become citizens in the far-eastern Indian state of Assam.

The Supreme Court had ordered the citizenship tally for Assam following years of campaigns against immigrants from Bangladesh who ethnic Assamese say are eating into their resources and taking their land.

A final register is due on July 31 but on Friday, Prime Minister Narendra Modi's government told the court that authorities needed to review the draft list of citizens because they believed many on it had given false information.

“There is a growing perception that ... illegal migrants have been included in the list in collusion with local officials,” government lawyer Tushar Mehta said.

Modi's Hindu nationalist Bharat Janata Party vowed to throw out illegal immigrants, labelling them termites during the campaign for this year's election it won with a huge mandate.

But the citizenship exercise has stoked deep anxiety among minority Muslims and risks further alienating them.

Critics have long accused BJP of a deep-seated bias against Muslims. The BJP denies the allegation but says it is opposed to the appeasement of any group.

“India cannot be the refugee capital of the world,” Mehta told the court.

Hundreds of thousands of people fled Bangladesh during its war of independence from Pakistan in 1971.

To be recognised as Indian citizens, all residents of Assam have had to produce documents proving that they or their families lived in the country before March 24, 1971.