Three Indian nationals have been arrested and charged over the murder of a Sikh separatist leader in Canada, which sparked a major diplomatic row between the two countries.

Hardeep Singh Nijjar, 45, was shot dead last June by masked gunmen in a busy parking lot in a Vancouver suburb.

The diplomatic row intensified after Prime Minister Justin Trudeau claimed the Indian government could be involved.

Delhi strongly denies the allegation.

Announcing the arrests on Friday, Superintendent Mandeep Mooker said the three suspects were Karan Brar, 22, Kamal Preet Singh, 22, and Karan Preet Singh, 28.

He said all three lived in Edmonton, Alberta, where they were arrested. They were charged with first-degree murder, according to court records, as well as conspiracy to commit murder.

All had been in Canada for three to five years, police said.

Police added that investigations were continuing, particularly into “links with the Indian government”.

“Separate investigations are underway into these matters. They are certainly not limited to the involvement of those arrested today,” said Assistant Commissioner David Teboul.

Investigators are working with their counterparts in India, but the collaboration has been “rather difficult and rather demanding” for several years, they said.

Police said there may be other people involved in the killing and there could be further arrests or charges.

Indian Foreign Minister Subrahmanyam Jaishankar said his country would wait for Canadian police to share information about those arrested, adding that the suspects “were apparently Indians from some sort of gang.”