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LONDON -- The diplomatic breach between Canada and India over the killing of a Sikh separatist leader near Vancouver has widened as both countries expelled one of the other's diplomats and India suspended visa processing for Canadian citizens.

Ties between the two countries, which are close security and trade partners and U.S. allies, strained after Canadian Prime Minister Justin Trudeau said on Monday that investigators were actively probing "credible allegations" about the potential involvement of Indian government agents in the killing of Hardeep Singh Nijjar.

India's Ministry of External Affairs quickly rejected Trudeau's allegations, calling them "absurd" and accusing Canada of sheltering "terrorists and extremists" who "continue to threaten India's sovereignty and territorial integrity," according to a statement.

Nijjar, who lived in Canada for over 20 years and advocated for Sikh independence while running a plumbing business, was gunned down on June 18 in front of a temple in Surrey, near Vancouver. In 2020, the Indian government had classified him as a terrorist belonging to a banned militant group, accusations that Nijjar and his followers always denied.

Canada is home to the largest Sikh population outside of India. According to Statistics Canada, the North American nation is home to 1.35 million Indians who make up around three percent of Canada's population.

In a notice posted on Thursday on BLS International, India's visa application center in Canada, the center announced it is suspending visa services for Canadians "until further" notice due to "operational reasons, with effect from 21 September 2023."

India's Ministry of External Affairs also issued an advisory for Indian Nationals and students in Canada, urging them to "exercise utmost caution" due to "growing anti-India activities and politically-condoned hate crimes and criminal violence in Canada."

ABC News has reached out to India's Ministry of External Affairs for comment.

"We are not looking to provoke or cause problems, but we are unequivocal about the rule of law and unequivocal about protecting Canadians and standing up for our values," Trudeau told reporters at the United Nations on Thursday.

"That is why we call upon the government of India to work with us to establish processes, to uncover the truth of the matter and allow justice and accountability to be served," he said.

Trudeau was in contact with U.S. President Joe Biden's administration about the findings on which he based his allegations before he made them public, a U.S. official told AP on condition of anonymity.

In response to the row, U.S. top national security adviser Jake Sullivan told reporters that there are "no exceptions" for actions like this.

"Regardless of the country, we will stand up and defend our basic principles and we will also consult closely with allies like Canada as they pursue their law enforcement and diplomatic process," Sullivan said.

Canada-India relations have grown significantly in recent years, with the Canadian government announcing that bilateral trade in goods reached nearly 12 billion Canadian dollars in 2023 - an increase of 57 percent from the previous year.

Now, pressure is mounting for the Canadian government to share more of the evidence for its "credible allegations," especially as some of the evidence reportedly came from Five Eyes allies, an intelligence-sharing network that includes the U.S., U.K., Canada, Australia and New Zealand, the CBC reported this week, citing Canadian government sources.

It's unclear whether the Nijjar question was already brought up at the recent G20 summit in New Delhi. A few days later, Canada announced it was cancelling a trade mission to India planned for the fall.

ABC News' Victoria Beaule contributed to this report.