A letter apparently from the Islamic State militant group threatening to kill Prime Minister Narendra Modi and Defence Minister Manohar Parrikar was received by Goa's State Secretariat last week, Press Trust of India (PTI) reported.

The letter, which expressed anger over ban on cow slaughter, has been handed over to the Goa Police, who have sent copies of it to all police stations in the state and also handed it over to the Anti Terrorist Squad (ATS).

The letter was written on a postcard and received last week by the State Secretariat.

"All the agencies of the state police are investigating this letter. We will soon be able to find the source of it," a senior police official told PTI.

"ISIS was written at the bottom of the postcard," he added.

With India playing host to French President Francois Hollande on its 67th Republic Day, the country is already vigilant in case of an attack attempt by Isis. Sources have warned India the Republic Day celebrations face "real threat" from Isis, reports NDTV.

Union Home Minister Rajnath Singh held a security review on 16 January, which chiefly discussed the problem of Isis' influence on the youth of the country and how a radicalised individual could attempt an attack during the French president's visit.