Canada's Prime Minister Justin Trudeau on Monday accused India of making a "fundamental error," as an escalating row over the killing of a Sikh separatist on Canadian soil last year saw both countries expel each other's ambassadors.

A grave Trudeau branded New Delhi's actions as "unacceptable" during a press conference in Ottawa on the diplomatic expulsions, which saw tensions reach a new height after the 2023 murder of Canadian citizen Hardeep Singh Nijjar.

Trudeau has previously said there were "credible allegations" linking Indian intelligence services to the killing. Both countries have traded barbs since then, culminating with the expulsions Monday of each other's ambassadors and five other top diplomats in both countries.

"The government of India made a fundamental error in thinking that they could engage in supporting criminal activity against Canadians here on Canadian soil, whether it be murders or extortion or other violent acts," Trudeau told reporters in Ottawa, hours after the expulsions were announced.

"We will never tolerate the involvement of a foreign government threatening and killing Canadian citizens on Canadian soil, a deeply unacceptable violation of Canada's sovereignty and of international law," he said.