The U.S. is raising concerns about India's part in a foiled assassination plot against Canadian-American Sikh separatist Gurpatwant Singh Pannun on American soil a few months ago.

National Security Council spokesperson Adrienne Watson said in a statement to The Hill: “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.”

Watson's statement also said that the council's Indian counterparts “expressed surprise and concern” about the alleged plot and said it would not be consistent with their policy.

It wasn't clear how the plot was thwarted, how the U.S. came to know about it, or who exactly was behind it. The FBI is investigating the incident.

US demands accountability

“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,” Watson said.

Pannun has not commented on the plot, and said he would let U.S. intelligence handle it.

“The foiled attempt on my life on American soil by the Indian agents is transnational terrorism which is a threat to the US sovereignty, freedom of speech and democracy, so I will let the U.S. government respond to this threat,” he said.

To thicken the plot even more, Pannun has been designated an "individual terrorist" by Indian authorities for threatening Air India and telling Sikhs their lives would be in danger if they flew on the airline.

Pannun countered that he was talking about a boycott, not a bomb, and accused the Indian authorities of trying to “crush freedom of expression.”

Rising tensions

Canada recently ramped up tensions with India after Canadian Prime Minister Justin Trudeau accused the Indian government of assassinating a Canadian citizen on its soil.

Hardeep Singh Nijjar was killed in September, and Trudeau thinks the Indian government had something to do with it.

Trudeau said there were "credible allegations" about India's involvement.

Nijjar had described himself as a “Sikh nationalist who believes in and supports Sikhs’ right to self-determination” before his death.

Meanwhile, the U.S. has been trying to improve relations with India as both nations face increasing threats from China.