Indian Prime Minister Narendra Modi condemned the blight of India's dwindling number of girls yesterday, launching a campaign to address the problem the United Nations has said amounts to a national emergency.

"In our neighborhood, girls are commonly killed in their mothers' wombs and we don't feel the pain," Modi said in a speech in the northern state of Haryana.

"We don't have a right to kill our daughters," he said.

Despite being banned, selective abortion is a growing problem that results in a steady decline in the number of girls being born even as the economy has grown and other social indicators like maternal mortality have improved.

The "Beti Bachao Beti Padhao" (Save the Daughter, Teach the Daughter) campaign is aimed at improving India's child sex ratio - the number of girls born for every 1,000 boys - and gender equality through access to education.

India's child sex ratio dropped from 964 in 1971 to a low of 918 in 2011, according to UN data. Between 2001 and 2011, the decline was seen in more than two-thirds of the districts in the nation.

A 2011 study published in the British medical journal the Lancet found that as many as 12 million Indian girls may have been selectively aborted between 1980 and 2010.

This trend, in a deeply patriarchal society where daughters can be seen as a financial liability, has led to an uptick in crimes including rape, trafficking and the emergence of "wife-sharing" in villages where the sex ratio is low, experts say.