The United States is treating an alleged plot to assassinate a Sikh separatist on American soil with "utmost seriousness" and has raised the issue with New Delhi, the White House said Wednesday.

The Financial Times reported that same day that US authorities had thwarted a conspiracy to kill Gurpatwant Singh Pannun, a US and Canadian citizen.

The row comes two months after neighboring Canada publicly linked Indian intelligence to the killing of another Sikh separatist.

"We are treating this issue with utmost seriousness, and it has been raised by the US government with the Indian government, including at the senior-most levels," US National Security Council spokeswoman Adrienne Watson said in a statement.

"Indian counterparts expressed surprise and concern. They stated that activity of this nature was not their policy."

Watson said the Indian government was believed to be "further investigating this issue and will have more to say about it in the coming days."

"We have conveyed our expectation that anyone deemed responsible should be held accountable."

Pannun is general counsel for Sikhs for Justice, a US-based group that is part of a movement calling for an independent Sikh state called Khalistan.

US President Joe Biden had raised the issue with Indian Prime Minister Narendra Modi during the G20 summit in New Delhi in September, the Financial Times reported.

Canada and India had a major diplomatic row after Canadian Prime Minister Justin Trudeau in September linked New Delhi to the killing of Canadian citizen Hardeep Singh Nijjar in June.

New Delhi called the Canadian allegations "absurd."