A Turkish Airlines flight from Bangkok made an emergency landing in New Delhi after a written bomb threat was found in a toilet on the Airbus A330 passenger jet carrying 134 passengers, the airline and Indian security sources said.

"Passengers have been asked to evacuate the plane, the drill is taking place right now," Hemendra Singh, spokesman for the federal Central Industrial Security Forces, told Reuters.

Turkish Airlines was targeted with a series of hoax bomb warnings in April and again over the weekend. This threat was found about three hours into the flight, a spokeswoman for the airline said.

Indian security forces said they were scouring the plane for signs of explosives. The flight will resume its journey to Istanbul if is given the all clear, the airline spokeswoman said.

The airliner was over the central Indian city of Nagpur when the pilot sent a message requesting an emergency landing and he was given permission to change course to Delhi, said Rajan Bhagat, spokesman for Delhi police.

The plane was being inspected in an isolated area of the airport, Bhagat said.