Reports of a new mass conversion of Christians in India have raised concerns over freedom of worship, days after Barack Obama challenged the country’s record on religious tolerance.

Details of the incident are unclear but it was reported that between 50 and 100 Christians from some of the poorest communities in India were “welcomed back” to Hinduism in a “homecoming ceremony” in a remote area in the eastern state of West Bengal on Wednesday.

A series of attempts by rightwing Hindu groups to hold mass conversion ceremonies have caused controversy in recent months. Conversion is illegal if there is any element of compulsion or bribery.

“The worry is that some kind of coercion is involved. The communities [involved in the recent incidents] are already vulnerable and the campaign seems quite aggressive and the combination is concerning,” said Meenakshi Ganguly, South Asia director of Human Rights Watch.

Last week the hardline group Vishnu Hindu Parishad (VHP) claimed to have “reconverted” more than 20 Christians in the southern state of Kerala. In December another group conducted a similar ceremony for more than 200 Muslims in the northern city of Agra.

The VHP appears to be behind Wednesday’s ceremony. “We are not committing any sin by bringing back our people to our own religion. This is a service to our country and we will continue with the ghar wapsi [homecomings],” Jugal Kishore, a VHP official involved in the event, was quoted as saying in the Indian Express newspaper.

Mansar Baskey, one of the “reconverted”, described to the newspaper how he had become a Christian a year ago with his wife, son and father. “I was told I have committed sin by converting to Christianity. Now, my family and I want to become Hindu again,” Baskey said.

Church authorities told the Indian Express that around 1,000 local people had been baptised in the poverty-stricken and remote area over the last year. Most are from the so-called tribal or adivasi community, among the most deprived in India.

However, other officials from the VHP denied to local reporters any reconversions, saying the ceremony had been to lay a foundation stone for a hostel, healthcare centre and school for the poor. Senior VHP functionaries contacted by the Guardian on Thursday said they were unable to comment.

Officials say an inquiry has been launched into Wednesday’s event. “Initial reports suggest those converted at the VHP rally are BJP workers who were lured. Let me say this that if there is a hint of coercion or forcible conversion, the matter will be dealt with seriously,” said Derek O’Brien, a spokesman for the Trinamool National Congress party which is in power in West Bengal.

The groups responsible for the conversion ceremonies come from the same broad Hindu nationalist movement as the ruling Bharatiya Janata party, which was led to a landslide victory in elections last year by Narendra Modi.

Modi, 64, a former organiser with a revivalist Hindu organisation, has yet to make any public comment on the reconversion ceremonies. However, he has warned members of parliament and ministers not to make statements that might distract from his development agenda, after one member of his government called Indian Muslims “bastard children” last year.

Hardliners’ plans for a mass ceremony in December were put on hold after pressure from senior officials within the movement and, it is thought, the government.

Obama’s words on Tuesday on the rights of religious minorities in the predominantly Hindu country came after three days of carefully choreographed demonstrations of warm relations between the US president and Modi.

“The peace we seek in the world begins in human hearts; it finds its glorious expression when we look beyond any differences in religion or tribe and rejoice in the beauty of every soul,” said Obama, who namechecked prominent Indian Muslims, Sikhs and sportswomen.

“No society is immune from the darkest impulses of men. India will succeed so long as it is not splintered along the lines of religious faith,” he added.

Modi has been criticised by opposition politicians for remaining silent on the issue of reconversion ceremonies. Digvijaya Singh, of the ousted Congress party, thanked Obama on Twitter for “speaking up for the Indian citizen’s rights to profess practice and propagate his religious belief”.

Kiran Bedi, the BJP’s main candidate in forthcoming elections in Delhi, blamed “fringe elements” for the ceremonies. “They are fringes. Fringes have been stopped, they have been given the message. The message has been conveyed in the party leadership’s own style,” Bedi said at the weekend.

Before becoming prime minister, Modi was previously denied a US visa following accusations that he stood by during, or even encouraged, sectarian violence in the western state of Gujarat in 2002, when he was chief minister. More than 1,000 people, mostly Muslims, were killed by rioters. Modi has denied any wrongdoing and a supreme court inquiry has found insufficient evidence to support the charges.

There are around 180 million Muslims in India, around 14% of the population of 1.3 billion. The Christian minority is significantly smaller.