Canadian police on Friday arrested and charged three Indian men with the murder of Sikh separatist leader Hardeep Singh Nijjar last year and said they were probing whether the men had ties to the Indian government.

Nijjar, 45, was shot dead in June outside a Sikh temple in Surrey, a Vancouver suburb with a large Sikh population. A few months later, Canadian Prime Minister Justin Trudeau cited evidence of Indian government involvement, prompting a diplomatic crisis with New Delhi.

Police said the matter was still under investigation and other probes were being carried out. The three men were named as Karanpreet Singh, 28, Kamalpreet Singh, 22 and Karan Brar, 22. 

"We're investigating their ties, if any, to the Indian government," Mandeep Mooker, a police superintendent, told a televised news conference.

Nijjar was a Canadian citizen campaigning for the creation of Khalistan, an independent Sikh homeland carved out of India. The presence of Sikh separatist groups in Canada has long frustrated New Delhi, which had labeled Nijjar a "terrorist".

Last week the White House expressed concern about the reported role of the Indian intelligence service in assassination plots in Canada and the United States.

Canadian police said they had worked with US law enforcement agencies, without giving additional details.

The trio, all Indian nationals, were arrested in the city of Edmonton in Alberta on Friday without incident, police said. The three are due to arrive in British Columbia by Monday.

Trudeau announced in September that Canadian authorities were pursuing allegations linking Indian government agents to the murder of Nijjar. New Delhi rejected Trudeau's claim as "absurd."

Canada had been pressing India to cooperate in its investigation. Last November, US authorities said an Indian government official had directed the plot in the attempted murder of Gurpatwant Singh Pannun, a Sikh separatist and dual citizen of the US and Canada.