Three United Nations human rights experts expressed “deep concern” on Thursday over a controversial citizens register in India's Assam state, warning it could inflame ethnic tensions in an already fractious region.

A new draft Register of Citizens (NRC) in the northeastern state announced in July left off four million people, leaving them potentially stateless and facing an uncertain future.

Critics say it is the latest move by the right-wing party of Prime Minister Narendra Modi to bolster India's Hindu majority at the expense of minorities.

India will hold a national election next year.

The policy was introduced by the state government, which is controlled by the same BJP party in power nationally.

Explore: How oppressed are Muslims in India?

“We are... seriously concerned about the lack of clarity regarding what will happen to those left out of the finalised NRC,” said a joint statement from the UN special rapporteur on religious freedoms, Ahmed Shaheed, the rapporteur for minority rights, Fernand de Varennes and an expert on arbitrary detentions, Seong-Phil Hong.

“There is a risk that persons not part of the NRC could become stateless, be at risk of deportation, or be subject to large-scale migration detention,” they said.

The deadline to provide the necessary documents to be included on the register has been set for December 31.

The current register includes only those who were able to prove they were in the state before 1971, when millions fled Bangladesh's war of independence into the state, and their descendants.

Assam, a state of 33 million people known for its lush tea estates, has for decades been racked by violence between indigenous tribes and settlers.

“It is feared that this entire process is increasing inter-ethnic tensions in a region that has already experienced a tumultuous history of identity-based conflicts,” de Varennes said.