India-Canada Tension: India has rejected the allegations as "absurd and motivated".
Soon after Canada Prime Minister accused India of being behind the killing of Khalistani terrorist Hardeep Singh Nijjar, the US said it is "deeply concerned" about the allegations.
"The US was deeply concerned about the allegations disclosed by Prime Minister Justin Trudeau", National Security Council spokesperson Adrienne Watson said in an emailed statement.
"It is critical that Canada's investigation proceed and the perpetrators be brought to justice," Ms Watson said.
Canadian Prime Minister Justin Trudeau said his government had "credible allegations" linking Hardeep Singh Nijjar's killing with the "agents of the Government of India."
India has rejected the allegations as "absurd and motivated".
"Such unsubstantiated" allegations seek to shift the focus from Khalistani terrorists and extremists, who have been provided "shelter in Canada and continue to threaten India's sovereignty and territorial integrity," the foreign ministry said in a statement.
India also expelled a senior Canadian diplomat today in a reciprocal move after Canada expelled a senior Indian diplomat.
"The High Commissioner of Canada to India was summoned today and informed about the decision of the Government of India to expel a senior Canadian diplomat based in India," the ministry said.
The Canadian diplomat, who is unnamed, has been asked to leave the country in five days.
Canada-based Nijjar was designated a 'terrorist' by India. He was shot dead by unknown attackers near a gurdwara in Surrey in June.
The new row sent relations between Ottawa and New Delhi, already sour, to a dramatic new low.
Earlier this month, Prime Minister Narendra Modi had expressed "strong concerns about continuing anti-India activities of extremist elements in Canada" during a meeting with his Canadian counterpart at the G20 Summit in Delhi.