The indictment of an Indian national in a foiled assassination attempt against a Sikh separatist on American soil underscored what Canada has been alleging in Hardeep Singh Nijjar's case and India must take it seriously, Prime Minister Justin Trudeau said.

Trudeau said Wednesday that Canadian authorities have been working closely with their American counterparts since August on allegations about the Indian government's involvement in Nijjar’s killing on June 18 in British Columbia province.

India had designated Nijjar as a terrorist in 2020. New Delhi has rejected Trudeau's allegations as "absurd" and "motivated".

The remarks by Trudeau came after Nikhil Gupta, 52, was charged on Wednesday with murder-for-hire in connection with his participation in a foiled plot to assassinate a US citizen in New York City.

“The news coming out of the United States further underscores what we’ve been talking about from the very beginning, which is that India needs to take this seriously. The Indian government needs to work with us to ensure that we’re getting to the bottom of this,” The Canadian Press quoted Trudeau as saying.

The indictment unsealed Wednesday in the US District Court for the Southern District of New York does not name the US citizen who was the target of the assassination plot.

However, The Financial Times, citing unnamed sources, last week reported that US authorities thwarted a plot to assassinate banned Sikhs for Justice’s Gurpatwant Singh Pannun, and issued a warning to the Indian government over concerns it was involved in the plot.

“This is not something that anyone can take lightly. Our responsibility is to keep Canadians safe, and that’s what we’re going to continue to do,” Trudeau emphasised.

On Canada's allegations, External Affairs Ministry spokesperson Arindam Bagchi said on Thursday that the main issue with Ottawa has been that of activities of anti-India elements in that country.

"In so far as Canada is concerned, we have said that they have consistently given space to anti-India extremists and violence and that is actually the heart of the issue. Our diplomatic representatives in Canada have borne the brunt of this," Bagchi said at a MEA briefing.

"We expect the government of Canada to live up to its obligations under the Vienna Convention on Diplomatic Relations. We have also seen interference by Canadian diplomats in our internal affairs," he said.

It is obviously unacceptable, Bagchi added.

Responding to the indictment of the Indian national, Canada's Public Safety Minister Dominic LeBlanc said Wednesday the US indictment “confirms that Canada is not alone at managing these particular threats.” “What’s important for us is the government of Canada and agencies like the Royal Canadian Mounted Police and the intelligence service do everything that they can to protect Canadians, but also to hold accountable those who murdered a Canadian citizen on Canadian soil," LeBlanc said.

Reacting to the development in the US, Canadian Foreign Minister Melanie Joly said that while she won’t comment on what’s happening in the US but said the Canadian government stands by its "credible allegations" that there was a killing of a Canadian on Canadian soil linked to Indian agents. She also urged India to engage in Canada's probe into Nijjar's murder.

"When it comes to what's happening in the US, I won't comment directly because, of course, I respect the work that the American law enforcement agencies are doing and I also respect the independence of their legal system," Joly said.

"What I can tell you though, is that we stand by our own credible allegations that there was a killing of a Canadian on Canadian soil, linking to Indian agents," she said in response to questions from journalists on Wednesday.

Joly said she had numerous conversations with her American colleagues, including Secretary of State Antony Blinken on the issue that Canada was facing with India.

"And at the same time, we call on India to engage in our own investigation, and I think it is important that they do so," she said.

She declined to comment on investigations into Nijjar's murder but said: "I can tell you the impact of our investigation, and also what is going on in the US, is I have engaged with my Indian counterpart on this very issue. We call on their cooperation to make sure that our investigation is able to proceed.

She said India's decision to remove diplomatic immunity to 41 Canadian diplomats was "completely unacceptable".

"And it is clearly my goal to make sure that the 41 diplomats that should be right now working in India are allowed back," she said.

India rejected Canada's attempt to "portray" the withdrawal of 41 Canadian diplomats from the country as a violation of international norms and asserted that ensuring two-way diplomatic parity is fully consistent with the provisions of the Vienna Convention on diplomatic relations.

The Ministry of External Affairs (MEA) in New Delhi had said the state bilateral relations, the much higher number of Canadian diplomats in India, and their continued interference in India's internal affairs warranted a parity in mutual diplomatic presence in New Delhi and Ottawa.

Canada and India witnessed strains in ties following Trudeau's allegations in September of the "potential" involvement of Indian agents in the killing of Nijjar.

India's High Commissioner Sanjay Kumar Verma last week said India was “absolutely” and “decidedly” not involved in the killing of Nijjar and Ottawa has "convicted" New Delhi even before the completion of the investigation.