The Maharashtra Anti-Terror Squad (ATS) has reportedly received an anonymous letter threatening to kill the Pune ATS officer who is handling investigating youths allegedly attempting to join Isis.

The letter, purportedly by the Islamic State group, has threatened to kill Pune-based ACP Bhanupratap Barge and his family, ANI reported.

The Maharashtra ATS received the letter about two weeks ago and have initiated a probe, Daily News & Analysis reported.

Barge had in December de-radicalised a 16-year-old girl from Pune who was influenced by Isis' ideologies after watching documentaries. She had begun to regularly follow Isis-related news on Al Jazeera news channel.

During investigation, it was revealed that she was planning to join the extremist group in Syria. It was also revealed that she was "ready to go to any extent to carry out the Isis directive," Barge had said last month, according to ANI.

"Later, she used the Internet to get in touch with Isis contacts and got in touch with about 200 young persons from different countries. Her interrogation revealed she was told to come to Syria for medical education and further initiation," DNA quoted him as saying.

Last month, a Mumbai-based youth, who had allegedly planned to join Isis, was detained by the ATS in Pune. He and his two friends, all residents of the Malwani suburbs in Mumbai, had been reported missing about three months ago. They were suspected to have joined Isis.

While the ATS detained 25-year-old Wajid Sheikh, 23-year-old Ayaz Sultan and 26-year-old Mohsin Sheikh are still missing. Several youths in India have been prevented from travelling to Syria to join Isis in the past.