Assam NRC: A road sign shows a new detention centre in Assam's Goalpara, 170km from Guwahati (AFP)

The controversial citizenship list in Assam that has left almost 19 lakh people facing statelessness has drawn criticism from the UN High Commissioner for Refugees.

Prime Minister Narendra Modi's BJP, which runs Assam where the National Register of Citizens (NRC) was collated, pushed for the list saying it was necessary to detect "foreign infiltrators".

Those left off have 120 days to appeal at special Foreigners Tribunals.

The UN High Commissioner for Refugees Filippo Grandi on Sunday called on the government to refrain from detaining or deporting anyone whose nationality is not verified through the process.

"Any process that could leave large numbers of people without a nationality would be an enormous blow to global efforts to eradicate statelessness," Mr Grandi added in a statement.

He urged the government to ensure no one ends up stateless, "including by ensuring adequate access to information, legal aid, and legal recourse in accordance with the highest standards of due process."

Assam has long seen large influxes from elsewhere, including under British colonial rule and around Bangladesh's 1971 war of independence when millions fled into India.

Under the NRC, only those who could demonstrate they or their forebears were in India before 1971 could be included in the list.

Assam villagers told AFP about family members who were excluded even though they had similar documents to their relatives.

"Our children's names are in the list but my wife's name is missing. She submitted all the documents and records... Why?" asked resident Jaynal Abudin.

Those left out, many of whom are poor and illiterate, have to navigate a long and expensive legal process that could include bringing their cases to the courts if they are rejected by a foreigner tribunal.

People rejected by the tribunals who have exhausted all other legal avenues can be declared foreigners and -- in theory -- be placed in one of six detention centres with a view to possible deportation.

The camps currently hold 1,135 people, according to the state government, and have been operating for years.

A senior government official told The Hindu newspaper on Sunday that it was not "practical" to send a large number of people excluded from the list to detention camps.

"No decision has been taken about the people who will be declared foreigners, maybe they are given work permits," the official, who was not named, told the newspaper.