India will identify and deport illegal immigrants from across the country, the Indian interior minister said on Wednesday, stepping up a campaign that critics say could stoke religious tension and further alienate minority Muslims.

An exercise to identify alien immigrants from Muslim-majority Bangladesh has been going on in the northeastern state of Assam for years, but Indian Prime Minister Narendra Modi's Hindu nationalist-led government has taken it up in earnest.

The campaign was a key issue in this year's general election, won by Narendra Modi's Hindu-nationalist party.

Indian Home Minister Amit Shah told parliament the government would not limit its efforts to Assam, but would come down hard on illegal immigrants anywhere.

“Illegal immigrants living on every inch of this country will be deported according to the law,” Shah told the upper house of parliament.

Shah, seen as a Hindu nationalist hardliner and a possible future replacement for Modi in the top job, called illegal migrants “termites” eating into Assam's resources during the election campaign.

Critics accuse Modi's Bharatiya Janata Party (BJP) of a deep-seated bias against minority Muslims and say the campaign against illegal migrants is aimed at Muslims, and threatens to further marginalise the community. The BJP denies the accusation and says it is opposed to the appeasement of any group.

While reinforcing measures against migrants slipping into the country, the government is trying to bring in a law that would simplify the process of getting Indian citizenship for immigrants from religious minorities persecuted in neighbouring Muslim countries, including Pakistan.

People in Assam are scrambling to prove their citizenship as part of an exercise to prepare a Supreme Court-ordered registry of citizens in the state. The list is due to be released on July 31.

A draft of the list released in July last year identified four million of the state's roughly 31 million people as illegal residents, including many Hindus.

But rights groups have warned that many residents, largely poor Muslims, are at risk of becoming stateless under the process. Other states in the northeast have launched similar exercises to identify people without Indian citizenship.

Mizoram state passed legislation in March to create separate registers for “residents” and “non-residents”, and the neighbouring state of Nagaland is working on a similar register.