The special aircraft carrying Prime Minister Manmohan Singh [Images] had a mid-air scare on Tuesday evening while preparing to land in New Delhi after the Air Traffic Control detected an unidentified blip on its flight path on the radar screen.

The Boeing 737 of the Indian Air Force landed safely though delayed by 16 minutes but not before the IGI Airport was put on high alert and security officials went in a tizzy.

As the PM's airplane, which was coming from Jamshedpur, was in its landing approach over Delhi airspace, the ATC suddenly noticed an unidentified target on the radar coming on the flight path of the VVIP plane, Airport Authority of India sources said.

An emergency drill on the ground was quickly put in place while air controllers asked the pilot of the aircraft to hover over the Delhi airspace for 16 minutes before clearing the plane for landing, AAI sources said. The pilot switched off the aircraft's Instrument Landing System and guided the plane on a changed flight path manually while making the final approach, sources said

As a result, Singh's arrival was delayed for some time. The aircraft landed on Runway 28 at the IGI Airport. Singh was in Jamshedpur in connection with an official function.

The AAI swiftly ordered a probe into the incident which raised serious security concerns over the Prime Minister's safety and investigators were trying to identify the blip.

When contacted, PMO sources declined to say anything.

Asked what the blip could mean, AAI sources said it could be an aircraft carrying a transponder on board, which could be linked to the secondary radars on ground. But some officials said this was not possible as it did not have a call sign, which any aircraft would normally have.

A systemic fault in the ATC system is also not being ruled out.

AAI officials said the blip could also have been a result of a bird flying at a high speed at a certain height. Another version for the blip being given was that it may be due to some sudden atmospheric changes.