The Washington Post reported that an officer in India's intelligence service was directly involved in a foiled plan to assassinate a U.S. citizen who is one of Indian Prime Minister Narendra Modi's most vocal critics in the United States. It said the officer was also involved in the separate shooting death of a Sikh activist last June in Canada.

India's foreign ministry said the Washington Post report made "unwarranted and unsubstantiated imputations on a serious matter" while New Delhi is investigating the issue.

"Speculative and irresponsible comments on it are not helpful," foreign ministry spokesperson Randhir Jaiswal said in a statement.

"This is a serious matter, and we're taking that very, very seriously," White House spokesperson Karine Jean-Pierre told reporters. "We're going to continue to raise our concerns."

In November, U.S. authorities said an Indian government official had directed the plot in the attempted murder of Gurpatwant Singh Pannun, a Sikh separatist and dual citizen of the United States and Canada.

India has expressed concern about the linkage and dissociated itself from the plot, saying it would formally investigate the concerns of the United States, and take 'necessary follow-up action' on the findings of a panel set up on Nov. 18.

Pannun is the general counsel of Sikhs for Justice, a group that India labeled an "unlawful association" in 2019, citing its involvement in extremist activities. Subsequently, in 2020, India listed Pannun as an "individual terrorist".

The issue is a delicate one for both India and the Biden administration in the United States as they try to build closer ties in the face of shared concerns about China's growing power.

News of the U.S. plot came two months after Canada said it was looking at credible allegations potentially linking Indian agents to the June murder of Hardeep Singh Nijjar, another Sikh separatist, in a Vancouver suburb.

India strongly rejected Ottawa's accusations.