US authorities have been in close contact with their Canadian counterparts about allegations the Indian government was involved in the murder of a Sikh separatist leader Hardeep Singh Nijjar in Canada and have urged India to cooperate with the investigation, a senior State Department official said on Tuesday.

“We have been in close contact with our Canadian colleagues about this. We’re quite concerned about the allegations. We think it’s important there is a full and open investigation and we would urge the Indian Government to cooperate with that investigation,” the official told reporters at a news briefing.

Canadian Prime Minister Justin Trudeau said authorities were “actively pursuing credible allegations” linking New Delhi’s agents to the murder of a Sikh separatist leader, an assertion India quickly dismissed as absurd.

The spat deals a fresh blow to diplomatic ties that have been fraying for years, with New Delhi unhappy over Sikh separatist activity in Canada. It now threatens trade ties too, with talks on a proposed trade deal frozen last week.

Each nation also expelled a diplomat in tat-for-tat moves, with Canada throwing out India’s top intelligence officer there while New Delhi responded by giving a Canadian diplomat five days to leave.

Hardeep Singh Nijjar: Who was the Sikh leader murdered in Canada?