India on Wednesday warned its citizens against visiting parts of Canada, the latest salvo in a diplomatic row over allegations New Delhi was involved in the killing of a Sikh separatist near Vancouver.

Canadian Prime Minister Justin Trudeau has demanded India treat with "utmost seriousness" the bombshell revelation of its probe into the murder.

The fallout prompted tit-for-tat diplomatic expulsions and a forceful denial from India, which said any suggestion it played a role in the June killing of Hardeep Singh Nijjar was "absurd."

Without explicitly referring to the row, India's foreign ministry said it was concerned for the safety of its citizens in Canada because of "politically condoned hate crimes and criminal violence."

"Threats have particularly targeted Indian diplomats and sections of the Indian community who oppose the anti-India agenda," a ministry statement said.

"Indian nationals are therefore advised to avoid traveling to regions and potential venues in Canada that have seen such incidents."

The advisory did not name specific cities or locations for Indians to avoid.

Nijjar was shot dead by two masked assailants outside the Sikh temple he presided over in Surrey, an outer suburb of Vancouver.

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.

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.