Canada expelled a top Indian diplomat Monday as it investigates what Prime Minister Justin Trudeau called credible allegations that India's government may have had links to the assassination in Canada of a Sikh activist.

Trudeau said in Parliament that Canadian intelligence agencies have been looking into the allegations after Sikh leader Hardeep Singh Nijjar, a strong supporter of an independent Sikh homeland known as Khalistan, was gunned down on June 18 outside a Sikh cultural center in Surrey, British Columbia.

Trudeau told Parliament that he brought up the slaying with Indian Prime Minister Narendra Modi at the G-20 last week. He said he told Modi that any Indian government involvement would be unacceptable and that he asked for cooperation in the investigation.

Canadian Foreign Minister Melanie Joly said the head of Indian intelligence in Canada has been expelled as a consequence.

'If proven true this would be a great violation of our sovereignty and of the most basic rule of how countries deal with each other,' Joly said. 'As a consequence, we have expelled a top Indian diplomat.'

The Indian Embassy in Ottawa did not immediately answer phone calls from The Associated Press seeking comment.

'Over the past number of weeks Canadian security agencies have been actively pursuing credible allegations of a potential link between agents of the government of India and the killing of a Canadian citizen, Hardeep Singh Nijjar,' Trudeau said.

Trudeau said Canada has declared its deep concerns to the Indian government. 'Any involvement of a foreign government in the killing of a Canadian citizen on Canadian soil is an unacceptable violation of our sovereignty.'

Trudeau said his government has been working closely and coordinating with Canada's allies on the case.

'In the strongest possible terms I continue to urge the government of India to cooperate with Canada to get to the bottom of this matter,' he said.

Trudeau said he knows there are some members of the Indo-Canadian community who feel angry or frightened, and he called for calm.

Public Safety Minister Dominic LeBlanc said Canada's national security adviser and the head of Canada's spy service have travelled to India to meet their counterparts and to confront the Indian intelligence agencies with the allegations.

He called it an active homicide investigation led by the Royal Canadian Mounted Police.

Joly said Trudeau also the raised the matter with U.S. President Joe Biden.

Relations between Canada and India have been tense in recent months. Trade talks have been derailed and Canada just canceled a trade mission to India that was planned for the fall.

Opposition Conservative leader Pierre Poilievre said if the allegations are true they represent 'an outrageous affront to our sovereignty.'

Opposition New Democrat leader Jagmeet Singh, who is himself Sikh, called it outrageous and shocking. Singh said he grew up hearing stories that challenging India's record on human rights might prevent you from getting a visa to travel there.

'But to hear the prime minister of Canada corroborate a potential link between a murder of a Canadian citizen on Canadian soil by a foreign government is something I could never have imagined,' Singh said.

The Khalistan movement is banned in India, where officials see it and affiliated groups as a national security threat. But the movement still has some support in northern India, as well as beyond, in countries like Canada and the United Kingdom which are home to a sizable Sikh diaspora.

The World Sikh Organization of Canada called Nijjar an outspoken supporter of Khalistan who 'often led peaceful protests against the violation of human rights actively taking place in India and in support of Khalistan.'

'Nijjar had publicly spoken of the threat to his life for months and said that he was targeted by Indian intelligence agencies,' the statement said.