NEW DELHI: The Hyderabad international airport was on Friday put on high alert after an anonymous letter threatening that explosives will be used and transported through a flight was received at the facility.

Sources said the Chief Airport Security Officer (CASO) of the sprawling complex received the letter which threatened that "explosives" will be used and transported through a flight which may take-off or land at the airport.

The letter was received at the the airport a day after Parliament approved a historic bill to carve out Telangana out of Andhra Pradesh amid vociferous protests by members from Seemandhra region as also from Trinamool Congress and Shiv Sena.

The letter does not clarify the handler or source of the explosives but states that the flight may come or go to Delhi, the sources said.

The Central Industrial Security Force (CISF), which guards the facility, has tightened security and increased its vigil even as a red-alert has been issued for the Rajiv Gandhi International Airport on the outskirts of the Andhra Pradesh capital.

CISF Additional Director General (Airport security) O P Singh confirmed the development to PTI but refused to elaborate.

Sources said the paramilitary has informed the Hyderabad Police about the development after which bomb squads and sniffer dogs have been deployed at the airport.

While the movement of fliers at the airport has not been restricted, frisking and scanning has been intensified and security personnel in mufti have been asked to keep an eye on suspicious movements.

The anonymous threat letter has been handed over to the local police which has tasked its special unit to find out the veracity and source of the letter.