New Delhi's Indira Gandhi International (IGI) airport is on high alert following an intelligence warning of a potential terrorist attack.
According to The Times of India, both Islamic State militants and the Taliban are planning to target Air India flights bound for Afghanistan.
Police reported that an anonymous telephone call, threatening that an Air India plane would be hijacked, was made to the Kolkata office of the state-owned carrier, on Saturday (3 December)
A senior officer of Kolkata Police said it was a brief call in Bengali threatening an attack on an Air India flight, but without specifying which flight would be targeted.
A special team has been trying to track the number from where the call was made to the national carrier's city office, a senior police officer said.
"After a complaint was lodged with the police yesterday, STF and detective department have been jointly looking into the case. Security has been tightened in the Air India office," the officer confirmed.
An Air India spokesman said appropriate security agencies like the Bureau of Civil Aviation Security (BCAS) are also looking into the issue. Senior officials of Bureau of Civil Aviation Security, CISF and AAI held a meeting in Kolkata on Saturday night to review the security measures.
The fresh alert - issued to the Bureau of Civil Aviation Security follows an earlier general alert issued in anticipation of US President Barack Obama's visit to India later this month. The US President is the chief guest at the Republic Day parade to be held in Delhi on January 26.
As security is heightened across the capital, the Central Industrial Security Force (CISF), which looks after the airport's internal security, has been warned to be alert to any suspicious behaviour.
Delhi Police, which is in charge of security outside the airport, was also put on alert.
Deputy Commissioner of police (IGI) MI Haider said: "Security has been extended and policemen have been increased."
CISF spokesperson Hemendra Singh said security has been tight for the last two months. "Flights going to Afghanistan are being monitored carefully," he confirmed
Security had already been increased at airports across the country and ground staff has been asked to thoroughly check all cabin baggage once the passengers boarded.
Al Qaeda has reportedly been trying to set up a base in the Indian subcontinent. In September, a video by the group's chief Ayman al-Zwahri surfaced, according to which Asim Umar, chief of Al Qaeda's Sharia Committee for Pakistan, has been named the leader of the new group - called the Qaedat al-Jihad.
India is currently training the Afghan National Army and is participating in the reconstruction efforts in the country. Indians in Afghanistan have also been frequent targets of militant attacks.