"An alert has been issued on the Indo-Nepal border following the arrest of an ISIS operative from Varanasi, Prime Minister Narendra Modi's parliamentary constituency on Wednesday," an official here said.

Sashastra Seema Bal (SSB) Assistant Commandant Lalit Mohan Doval said on Friday that the alert had been sounded on the Sonauli border in the district.

"Every passing citizen is being thoroughly screened and the jawans have been instructed to remain alert, he added. Permission to enter India is being issued only after intensive investigation and inquiry at the Sonauli main gate," Doval said.

"SSB personnel are keeping a close watch on the routes from Bhagwanpur, Shyamkot, Danda Head, Khanua, Hardidali, Sundi, Mudila, Chandithan and Sampatiha villages. People acting in a suspicious manner are being questioned and allowed to cross the border only after their identities are confirmed," Doval said.

Another senior official said officers from intelligence agencies are camping on the border while the 22nd and 66th battalions of the SSB had increased patrolling.

A "highly radicalised" operative of the ISIS' 'Voice of Hind' module was arrested after the National Investigation Agency raided two locations in Uttar Pradesh and Delhi on Wednesday.

Basit Kalam Siddiqui (24), a resident of Varanasi in Uttar Pradesh, was arrested in a case related to a conspiracy by the proscribed terrorist organisation to radicalise and recruit youngsters to wage violent jihad against the Indian State by carrying out acts of terrorist violence, an agency spokesperson had said.