NEW DELHI - India's aviation watchdog has launched an inquiry after reports that a Jet Airways plane suddenly dropped 5,000ft over Turkey's crowded airspace when its pilot was resting and the co-pilot was looking at her tablet computer.

"The commander was taking controlled rest, which means he was sleeping as per global airline norms where pilots take rest in the cockpit by turn on long flights,'' Times of India quoted a source who spoke to the two pilots of the Boeing 777 aircraft.

"The co-pilot, who was supposed to ensure that the aircraft flew steadily on its assigned path, told....investigators that she was on her electronic flight bag (EFB) - a tablet that has all aircraft documents loaded on to it. She did not realise that the aircraft had dropped below its assigned level," the source said.

The incident occurred last Friday when the aircraft was travelling from Mumbai to Brussels.

The co-pilot was unaware of the situation until she received an emergency call from an air traffic controller in Ankara asking why the aircraft, which can hold almost 300 passengers, had dropped out of its assigned flight level of 34,000 feet.

Daily Mail reported that other aircraft were flying at the altitude at that time and the airspace over Turkey is busier than normal as airlines avoid conflict zones in Iraq and Ukraine.

The co-pilot woke up the pilot when the air traffic controller asked them to immediately ascend to 32,000ft, said the report. The crew did not report the incident to India's Directorate General of Civil Aviation (DGCA) but the regulator was alerted when it received an anonymous text message about the uncontrolled descent.

The pilot and co-pilot have been suspended pending the outcome of the investigation.

"Was some button pressed wrongly and what was the co-pilot doing - whether she was watching something on the tablet or she too had dozed off - needs to be probed," the Times of India quoted a senior DGCA official as saying.