WASHINGTON (AP) — The U.S. has thwarted a plot to kill Sikh separatist leader Gurpatwant Singh Pannun on American soil, a U.S. official familiar with the matter said Wednesday.

U.S. authorities have raised concerns with New Delhi that the Indian government may have had knowledge of the plot, according to the official who was not authorized to comment on the sensitive matter.

The official declined to comment on when or how U.S. officials became aware of the plot as well as how the alleged assassination attempt was derailed. The FBI is investigating the matter, the official said.

Spokespeople for the FBI and Justice Department declined to comment Wednesday.

The revelation follows the September disclosure by Canadian Prime Minister Justin Trudeau of what he said were credible allegations that the Indian government may have had links to the assassination in that country of Sikh activist Hardeep Singh Nijjar. India rejected the accusation as absurd, but Canada expelled a top Indian diplomat while it investigated.

The thwarted assassination plot was first reported by the Financial Times, which said the U.S. informed some allies of the alleged plot.

The official who spoke to AP added that concerns over the plot have been raised by U.S. authorities at the highest levels of the Indian government, and officials in New Delhi indicated they were treating the matter seriously.

“We are treating this issue with utmost seriousness, and it has been raised by the U.S. Government with the Indian Government, including at the senior-most levels. Indian counterparts expressed surprise and concern,” National Security Council spokeswoman Adrienne Watson said in a statement.

“They stated that activity of this nature was not their policy. Based on discussion with senior U.S. government officials, we understand the Indian government is 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," the statement added.

Law enforcement officials have repeatedly warned about what they say is an alarming trend of foreign governments seeking to harass, stalk or intimidate dissidents and political opponents in the U.S.

India's foreign ministry issued a statement noting that the U.S. had raised information pertaining to a “nexus between organized criminals, gun runners, terrorists and others” during recent conversations. The statement described the information as a “cause of concern for both countries” and said India takes it seriously.

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Associated Press writer Eric Tucker in Washington contributed to this report.