A massive combing operation is in progress against NDFB-S militants three days after they killed at least 75 people in the simultaneous serial attacks in Sonitpur and Kokrajhar districts. Comprising army, CRPF, Assam Rifles and police, the combing operation began in Sonitpur district in the wee hours. Security forces are concentrating on the areas along the Assam-Arunachal Pradesh border which has been the safe escape route for the militants. Similar operation is on the cards along the Indo-Bhutan border in Kokrajhar district where the militants have killed at least 25 people on Tuesday evening forcing the authority to clamp curfew in the vulnerable and sensitive areas. The intensive crackdown took place hours after Union home minister announced zero tolerance to the murder and mayhem by the militant outfit and refusing to accept any offer for talks in the present situation.