Federal prosecutors have charged a man they identified as an Indian intelligence officer with trying to orchestrate from abroad an assassination on U.S. soil — part of an escalating response from the U.S. and Canada to what those governments see as brazenly illegal conduct by a longtime partner.

An indictment unsealed in Manhattan on Thursday said that the man, Vikash Yadav, “directed the assassination plot from India” that targeted a New York-based critic of the Indian government, a Sikh lawyer and political activist who has urged the Punjab region of India to secede.

The target of the New York plot has been identified by American officials as Gurpatwant Singh Pannun, the general counsel of Sikhs for Justice.

In a statement, Mr. Pannun called the plot to kill him a “blatant case of India’s transnational terrorism which has become a challenge to America’s sovereignty and threat to freedom of speech and democracy.”

The indictment said that Mr. Yadav called himself a “senior field officer” in the part of the Indian government that includes its foreign intelligence service, known as the Research and Analysis Wing, or RAW.

Authorities say Mr. Yadav recruited an associate to find a U.S.-based criminal to arrange the murder of the Sikh activist. Last year, U.S. prosecutors charged the man accused of being Mr. Yadav’s henchman, Nikhil Gupta, and said Mr. Gupta had acted under instructions from an unidentified employee of the Indian government. Now, prosecutors have charged Mr. Yadav with orchestrating the plot.