The US has said it foiled an alleged plot to assassinate an American citizen in New York who advocated for a Sikh separatist state.

Nikhil Gupta, an Indian national, is accused of trying to hire a hitman for $100,000 (£79,000) in cash.

But the hitman was actually an undercover federal agent, prosecutors said.

Mr Gupta, 52, is in jail in the Czech Republic pending extradition. The charges carry up to 20 years in prison.

He was allegedly directed by an Indian government official who was not named or charged in the indictment.

The White House said it had raised the alleged assassination plot with India at the most senior levels, and officials there responded with "surprise and concern".

A senior administration official said the murder-for-hire allegations so concerned President Joe Biden that he dispatched top US intelligence leaders - CIA Director William Burns and Director of National Intelligence Avril Haines - to India to speak to officials there.

India's government, meanwhile, said it was investigating and had formed a high-level inquiry committee to "address the security concerns highlighted by the US government".

While the target was not named by prosecutors, US media reported it was Gurpatwant Singh Pannun, a dual US-Canadian citizen and member of a US-based Sikh separatist group.

"Is India ready to face the consequences of using trans-national terrorism?" Mr Pannun said to the BBC's Asian Network about the alleged plot.

He added that he was sure anyone who was "attempting to kill or assassinate me - whether it is India diplomats, whether it is Indian RAW [Research and Analysis Wing intelligence] agents - they will face the law".

India has designated Mr Pannun a terrorist - an allegation he denies.

India says he repeatedly issues threats against the country. But Mr Pannun says he is an activist who just believes in the Khalistan movement.

Sikhs are a religious minority that make up about 2% of India's population. Some groups have long called for a separate homeland for Sikhs called Khalistan.

Those calls peaked in India in the 1980s with an armed insurgency that was later crushed, killing thousands. Supporters in the Sikh diaspora have continued to advocate for Khalistan, however.

The movement is not prominent in India now with most major political parties, including in Punjab, opposing such calls.

News of the alleged plot in the US comes after Canada's Prime Minister Justin Trudeau said there was "credible" evidence tying the Indian government to the murder of another Sikh leader in British Columbia in June. India denies any role in that killing.

Mr Trudeau said the indictment on Wednesday underscored the need for India to take that allegation seriously.

"We will hit all our targets," Mr Gupta allegedly wrote in a text message to the official.

The indictment alleges Mr Gupta suggested posing as a client for the target so he could "lure [him] to a place where he could be more easily executed".

It says he intended to meet a hitman in New York City. Instead, he was introduced to an undercover law enforcement officer who said he would carry out the killing for $100,000.

Mr Gupta paid $15,000 upfront via an associate on 9 June, prosecutors said, and the indictment includes an apparent photograph of this payment.

He was arrested later that month in the Czech Republic where he is awaiting extradition to the US.

The target was also an associate of Hardeep Singh Nijjar, the leader in the Sikh independence movement who was murdered by a masked gunman in Canada in June.

Shortly after Mr Nijjar's murder, Mr Gupta allegedly received a video clip from the Indian official that showed Mr Nijjar's "bloody body slumped in his vehicle".

The indictment says the Indian official then sent the target's address to Mr Gupta.