India's anti-terror agency has filed a case against a Sikh separatist leader for threatening not to let flag carrier Air India operate anywhere in the world, while warning its passengers of danger to their lives.

The agency said security forces were on alert after the threats by Gurpatwant Singh Pannun, who acts as general counsel of Sikhs for Justice (SFJ), a group campaigning to establish an independent Sikh homeland called Khalistan carved out of India.

The case against Pannun has been registered under various provisions of the Unlawful Activities (Prevention) Act 1967 and under sections of the Indian Penal Code, the National Investigation Agency (NIA) said in a statement on Monday.

"Pannun threatened that Air India would not be allowed to operate in the world ... in his video messages, released on Nov. 4," it said, adding that he had urged Sikhs not to travel on Air India flights from Sunday, "claiming a threat to their lives".

Reuters has not independently verified the video messages, which were widely shared on social media this month.

Pannun's Sikhs for Justice group did not immediately respond to a request for comment sent to an email address listed on its website. Air India did not immediately respond to a request for comment.

India banned the SFJ as an "unlawful association" in 2019 and listed Pannun as an "individual terrorist" in 2020.

Pannun is a resident of the United States, the NIA said that year. Media have said he has dual citizenship of the United States and Canada. The SFJ has offices in Britain, Canada and the United States, it says on its website.

The threats come as Canadian agencies investigate "credible" allegations linking Indian government agents to the June killing there of a Sikh separatist leader, which frayed ties between the two countries. India has rejected Canada's suspicions.

In the wake of the threats, investigations have been launched in Canada, India and some other countries where the airline, owned by the Tata Group conglomerate, operates, the NIA said in its statement.

Pannun has also previously threatened to disrupt railways and thermal power plants in India, the agency said.

Interpol has rejected two requests by India to issue a red corner notice against Pannun, the Indian Express newspaper said in October last year.

Air India has previously been targeted by Sikh militants, who were blamed for a bombing in 1985 of its Boeing 747 aircraft flying from Canada to India that killed all 329 people aboard off the Irish coast.

The demand for Khalistan has resurfaced many times, although it now has little support in India, which sees the movement as a security threat.

A violent insurgency in the 1970s and 1980s by Sikh militants paralysed the northern state of Punjab, where Sikhs are a majority, for more than a decade. (Reporting by Shivam Patel; Editing by Clarence Fernandez)