Prime Minister Justin Trudeau on Tuesday demanded that India treat with "utmost seriousness" Canada's allegations of New Delhi's possible involvement in the slaying of a Sikh exile, a concern echoed by Washington.

Canada's assertion led to reciprocal expulsions of an Indian intelligence official from Canada and a senior Canadian diplomat from New Delhi.

"India needs to take this matter with the utmost seriousness. We are doing that, we are not looking to provoke or escalate," Trudeau told reporters.

Canada said Monday that there were "credible allegations" that agents linked to New Delhi were responsible for the murder June 18 of Hardeep Singh Nijjar, a Canadian citizen, in front of a Sikh cultural center in a Vancouver suburb.

The government of Indian Prime Minister Narendra Modi said the leader had "completely rejected" Canada's assertions in the unsolved slaying.

"Allegations of Government of India's involvement in any act of violence in Canada are absurd," the foreign ministry said in a statement.

Washington, however, joined Ottawa in calling for India to reveal what it knows about the slaying.

"We are deeply concerned about the allegations referenced by Prime Minister Trudeau," National Security Council spokeswoman Adrienne Watson said in a statement.

"We remain in regular contact with our Canadian partners. It is critical that Canada's investigation proceed and the perpetrators be brought to justice."

Sikh nationalism 

An activist for the creation of a Sikh state known as Khalistan, Nijjar was wanted by Indian authorities for alleged terrorism and conspiracy to commit murder.

He had denied those charges, according to the World Sikh Organization of Canada, a nonprofit organization that says it defends the interests of Canadian Sikhs.

Relations between Canada and India have been strained in recent months since the assassination of the Sikh leader and demonstrations that followed in Canada.

The Indian government accuses Ottawa of turning a blind eye to the activities of radical Sikh nationalists who advocate the creation of an independent Sikh state to be carved out of northern India.

British Foreign Secretary James Cleverly, who was at the United Nations, said his nation wants India's "full cooperation" with Canada in the probe.

Trudeau said the case is "extremely serious" and "has far reaching consequences… for Canada."

Trudeau said his government did not rush to judgment in the case and had worked closely with its intelligence agencies.

"We wanted to make sure that we had a solid grounding in understanding what was going on and analysis and indeed in facts," he said, adding that Ottawa had "fully shared with the government of India the seriousness… of our preoccupations and indeed conclusions."

'No surprise to Sikhs' 

A representative of the World Sikh Organization of Canada, Mukhbir Singh, said his countrymen may have been "shocked" by Trudeau's assertion "but it was no surprise to the Sikh community."

"For decades, India has targeted Sikhs in Canada with espionage, disinformation and now murder," he alleged.

A Sikh lawyer in the Toronto area, Harkirt Singh Dhadda, said Sikhs want to see "those who pulled the trigger and the ones who plotted this assassination" brought to justice.

In a sign of the simmering crisis, Ottawa recently suspended negotiations for a free-trade agreement with India, and last week the minister of trade canceled a trip to the country planned for October.

In contrast, the US government's relations with India have steadily been upgraded as Washington views New Delhi as a key ally in countering Chinese influence in the Indo-Pacific region.

President Joe Biden rolled out the red carpet in June for Modi, hosting him in only the third state visit of his presidency.