• Tensions escalate after Canada, India expel each other's ambassadors.
  
• US wants to see India cooperate with Canada in investigation.
  
• Washington sees India as natural partner in face of rising China.
  

The United States on Tuesday urged India to take seriously Canada's allegations of an assassination plot, following the expulsion of each other's ambassadors by India and Canada, two US partners.

As tensions between the countries escalate, State Department spokesman Matthew Miller emphasised the importance of cooperation between the two counteries as he spoke to reporters on Tuesday.

"When it comes to the Canadian matter, we have made clear that the allegations are extremely serious and they need to be taken seriously. And we wanted to see the government of India cooperate with Canada in its investigation," Miller told reporters.

"Obviously, they have not," he said. "They have chosen an alternate path."

Canada accused India of involvement in a campaign against Sikh separatists that went beyond what was previously known, with Prime Minister Justin Trudeau saying New Delhi had made a "fundamental error."

The dispute intensified after Canada alleged that the Indian government was involved in last year's killing outside a Sikh temple of Hardeep Singh Nijjar, an advocate for an independent Sikh state who had immigrated to Canada and become a citizen.

The US has alleged a similar, albeit unsuccessful, assassination plot by India on US soil but has handled the matter more quietly.

An Indian "Enquiry Committee" formed in response to the US allegations was visiting Washington on Tuesday to discuss the case, the State Department said.

India "has informed the United States they are continuing their efforts to investigate other linkages of the former government employee and will determine follow up steps, as necessary," the State Department said.

National Security Council spokesman John Kirby said of India: "The fact that they sent an Enquiry Committee here, I think, demonstrates that they are taking this seriously."

The US has been courting India for more than two decades, seeing it as a natural partner in the face of a rising China, despite rights groups' charges of a closing democratic space under Prime Minister Narendra Modi.