They allege the Indian government employee, who worked in security and intelligence, recruited another Indian national named as Nikhil Gupta to plan the assassination of a US citizen who had advocated for a Sikh sovereign state in northern India.

It comes after the Canadian government sparked a diplomatic row by claiming India may have been involved in the June killing of Sikh separatist leader Hardeep Singh Nijjar in British Columbia.

On Wednesday, US Department of Justice (DoJ) officials announced charges against Gupta, 52, who they claim worked with the Indian government employee on the alleged plot to kill the US citizen.

The Indian government official is named only as CC-1 but has described himself as a "senior field officer" with responsibilities in "security management" and "intelligence" and also claims to have served in India's Central Reserve Police Force and been trained in "battle craft" and "weapons," an indictment against Gupta said.

Prosecutors allege the Indian official recruited Gupta in May 2023 to orchestrate the assassination, and he then reached out to someone he believed was a criminal associate for help hiring a hitman.

However, prosecutors said the associate was actually an undercover agent with the Drug Enforcement Agency (DEA).

According to the indictment, released on Wednesday, Gupta agreed to a $100,000 (£79,000) fee for the killing and paid $15,000 (£12,000) upfront via an associate.

The target of the assassination was not named in the court documents, but the Associated Press identified him as Gurpatwant Singh Pannun - a US-based Sikh separatist whom the Indian government considers a terrorist.

The indictment said that, in June, the Indian government employee gave Gupta the home address of Pannun, his phone numbers and details about his daily conduct, including surveillance photographs, which Gupta then passed on to the undercover DEA agent.

It added that Gupta told the undercover agent to carry out the murder as quickly as possible, but also warned the agent not to act during meetings between US and Indian officials.

US Attorney Damian Williams said: "The defendant conspired from India to assassinate, right here in New York City, a US citizen of Indian origin who has publicly advocated for the establishment of a sovereign state for Sikhs, an ethnoreligious minority group in India."

Gupta, who faces two counts of murder-for-hire and murder-for-hire conspiracy, was arrested by Czech authorities in June and is awaiting extradition.

The news agency Reuters says he could not be reached for comment.

The charges come after an official of President Joe Biden's administration last week said authorities had thwarted a plot to kill Pannun.

The official also issued a warning to India over concerns the government in New Delhi may have had knowledge of the plot.

On Wednesday, India's foreign ministry said New Delhi would formally investigate the concerns aired by the US.

"India takes such inputs seriously since they impinge on our national security interests as well," the ministry said, vowing to "take necessary follow-up action" on the findings.

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The Indian government has complained about the presence of Sikh separatist groups outside India, including in Canada and the US.

The groups have kept alive the movement for Khalistan - the demand for an independent Sikh state to be carved out of India.

The movement is considered a security threat by India - who have blamed Sikh militants for the 1985 bombing of an Air India Boeing 747 flying from Canada to India when all 329 people on board were killed.

In October, Canada's prime minister, Justin Trudeau, said there were "credible allegations" that India was involved in the killing of Nijjar.

The Indian government accused Nijjar in 2020 of being a leader of a Sikh independence militant group, reported Reuters, which also said New Delhi claims he was associated with a "terrorist organisation".

Nijjar, a 45-year-old Canadian citizen, was shot dead outside a Sikh temple in Surrey, British Columbia, on 18 June.

Claims it was involved in Nijjar's death were dismissed as "absurd" by India, which threatened to revoke the diplomatic immunity of Canadian diplomats following Mr Trudeau's comments.

Canada responded by withdrawing 41 of its 62 diplomats from India. It also expelled an Indian diplomat from Ottawa over the affair.

Mr Trudeau has responded to the latest allegations against Gupta by saying that the US investigation underscores the need for India to take similar allegations by Canada seriously.

The White House - which has placed a high priority on improving ties with India to counter China - declined to comment directly on the charges against Gupta, but said administration officials acted quickly.

White House National Security Council spokesperson Adrienne Watson said in a statement: "When we were made aware of the fact that the defendant in this case had credibly indicated that he was directed to arrange the murder by an individual who is assessed to be an employee of the Indian government, we took this information very seriously and engaged in direct conversations with the Indian government at the highest levels to express our concern."