The US is in touch with Indians at high levels after Ottawa said Indian government agents had links to the murder of a Sikh separatist leader in Canada, and Washington is giving India no "special exemption" in the matter, US national security adviser Jake Sullivan said on Thursday.

The United States has been seeking to strengthen its relationship with India. President Joe Biden hosted Indian Prime Minister Narendra Modi for a state visit at the White House earlier this year.

Asked whether US concern over the incident could disrupt that process, Sullivan said the United States would stand up for its principles, regardless of what country is affected.

"It is a matter of concern for us. It is something we take seriously. It is something we will keep working on, and we will do that regardless of the country," Sullivan told reporters at the White House.

"There's not some special exemption you get for actions like this. Regardless of the country, we will stand up and defend our basic principles and we will also consult closely with allies like Canada as they pursue their law enforcement and diplomatic process."

Canada said on Monday that it was "actively pursuing credible allegations" linking Indian government agents to the murder of Hardeep Singh Nijjar, 45, outside a Sikh temple in June.

Sullivan noted that the United States was in touch with both countries about the topic.

"We are in constant contact with our Canadian counterparts ... and we have also been in touch with the Indian government,” Sullivan said.

Sullivan said he disagreed with reports suggesting there was distance between Canada and the United States on the issue.

"I firmly reject the idea that there is a wedge between the US and Canada. We have deep concerns about the allegations and we would like to see this investigation carried forward and the perpetrators held to account," he said.