The unprecedented move signals growing international pressure on India over the controversial law

The United Nations High Commissioner for Human Rights (UNHCR) Michelle Bachelet has approached the Indian Supreme Court over "the exclusions of persons... on the basis of their religion" from the controversial Citizenship Amendment Act (CAA).

In her intervention plea, the High Commissioner asked to be made party in a case against the law that is being heard by the top court, reports NDTV.

Bachelet said the "differentiations" drawn by the law are not "sufficiently objective and reasonable".

The Indian External Affairs Ministry hit back with a sharply-worded statement.

It declared the citizenship law as an "internal matter" and saying "no foreign party had any locus standi on issues pertaining to India's sovereignty".

"The Citizenship Amendment Act is an internal matter and concerns the sovereign right of the Indian parliament to make laws. We strongly believe no foreign party has any locus standi on issues pertaining to India's sovereignty," the ministry said on Tuesday.

The ministry's response also said the citizenship law is "constitutionally valid" and upholds human rights values. Indian government says the law will help non-Muslim refugees fleeing religious persecution from Muslim-dominated neighbouring countries.

"It is reflective of our long-standing national commitment in respect of human rights issues arising from the tragedy of the Partition of India," the ministry statement added.

The government's claim that the law will help non-Muslim refugees because Islam is the state religion in the countries listed - Afghanistan, Bangladesh and Pakistan, has been disputed by the petition.

"Recent reports... ascertain there exist a number of religious groups considered religious minorities in these countries, especially of the Muslim faith, including Ahmadi, Hazara and Shia Muslims whose situations would warrant protection on the same basis as that provided in the preferential treatment proposed by the CAA," the petition stated.

The Supreme Court is currently hearing a massive 143 petitions challenging the legal validity of the new citizenship law. In a January hearing the court declined to put the law on hold and, instead, gave the central government four weeks to respond.

Indian parliament passed the law in December and has been fiercely criticised by the opposition and activists as being anti-Muslim and violating secular tenets of the Indian constitution.

It is also feared that the law, used with the NRC (national register of citizens) and NPR (national population register) can be used to further target Muslims.

Sustained nationwide protests, some of which have been led by students and women, have broken out since it was passed, with lakhs taking to the streets. A peaceful and weeks-long protest in Delhi's Shaheen Bagh has emerged as the epicentre of these protests.

Protests against the law have drawn the attention of students in foreign countries and celebrities like Pink Floyd co-founder Roger Waters, who this week called the CAA "fascist".

Last week, horrific violence took place in Delhi when a group attacked the protesters. 48 people were killed and hundreds were injured in the violence.

Shortly after it was passed the United Nations expressed concern over a law that it said was "fundamentally discriminatory in nature".

A spokesperson for the Office of the UN High Commissioner for Human Rights (OHCHR) said it would "have a discriminatory effect on people's access to nationality".

Prime Minister Narendra Modi, Home Minister Amit Shah and other Bhartiya Janta Party (BJP) leaders have repeatedly defended the bill, claiming it will help refugees. They say the opposition is spreading rumours and misinformation about the law for political gain.

In several non-BJP ruled states, such as Bengal, Kerala and Punjab, the law has been opposed. These states have passed resolutions against the law and, in the case of the first two, Bengal and Kerala have stopped working on NPR and NRC.