The dwindling numbers of Indian girls, caused by the illegal abortion of millions of babies, has reached "emergency proportions", fuelling an increase in crimes such as kidnapping and trafficking, the United Nations warned on Tuesday.

Despite laws that ban expectant parents from running tests to determine the gender of unborn children, female foeticide remains a common practice in parts of India, where a preference for sons runs deep.

"It is tragically ironic that the one who creates life is herself denied the right to be born," said Lakshmi Puri, deputy executive director of U.N. Women, at the launch of a new study on sex ratios and gender-biased sex selection.

India's traditionally male-dominated culture views sons as assets -- breadwinners who will provide for the family, carry on the family name, and perform the last rites for their parents, an important ritual in many faiths.

Girls, however, are often seen as a liability, with families having to dig deep for a substantial dowry to ensure a desirable match. In a culture that views pre-marital sex as bringing shame to the girl's family, parents also worry about their safety.

India's 2011 census showed that while the overall female-to-male ratio has improved marginally since the last census a decade ago, fewer girls were born than boys and the number of girls younger than six plummeted for the fifth straight decade.

"The sharply declining child-sex ratio in India has reached emergency proportions and urgent action must be taken to alleviate this crisis," Puri added.

A May 2011 study in British medical journal the Lancet found that up to 12 million Indian girls were aborted over the last three decades, resulting in a skewed child sex ratio of 918 girls to every 1,000 boys in 2011, versus 962 in 1981.

Activists blame ultra-sonography for the rise in abortions, saying the technology is used for sex determination.

But the crime is tough to check, they add, resulting in few convictions. There were 221 cases of foeticide reported in 2013, up from 210 in 2012, the National Crime Records Bureau says.

U.N. officials said India's economic and social progress had failed in the area of sex selection, and the unbalanced sex ratio was contributing to crimes such as rape, abduction and trafficking.

The entire social structure will have to change, with a battle waged against the root causes of a preference for sons, said Lise Grande, the U.N. resident coordinator in India.

"This may be one of the hardest, most difficult struggles India faces, but it is arguably one of its most important," she added.