The incident happened at about 5-6 p.m. on Saturday. Police have arrested two Americans - a father-son duo - in this connection, Deputy Commissioner of Police, New Delhi, Eish Singhal told IANS.

Police have yet to establish the Americans' motive behind the flying drone in such sensitive and high-security zone, despite interrogating both of them till late night on Saturday.

"Their motive behind flying the drone is not clear. Investigation is going on," Singhal said.

A Delhi Police officer, on the condition of anonymity, told IANS: "The matter relates to flying a drone in a very sensitive zone. The arrested accused are Americans. Intelligence officials are unlikely to say anything on the issue. As a precautionary measure, we have registered a case at South Avenue police station for record."

According to information available to IANS, a passerby saw the drone flying over the Rashtrapati Bhawan and the PMO and informed the police. IANS has exclusive access to the phone number from which the phone call was made.

Police thought it was a hoax call, but as the matter pertained to a high security area, they sent a police constable to South Avenue to see what is happening.

The constable saw the flying drone, immediately informed his senior officers and tried to find out who was behind it.

Almost all top officers of Delhi Police reached the spot but by then the constable had detained two Americans whom he saw controlling the flying drone.

Both Americans were brought to the police station for interrogation, and intelligence agencies alerted about the incident.

During the interrogation, the Americans admitted that they were flying the drone and claimed they were using it to shoot a video for a portal they worked for. They further said that they had no idea that drones are banned in this area.

However, police are reluctant to accept their explanations.

According to sources, both the Americans were in Delhi for the last four days, and staying in the Maurya Sheraton hotel.

The father, aged 50, is a horticulturist in America while his son, aged 30 is a professional videographer.

"They have been released for the time being. We have confiscated the drone. Investigation into the matter is on," said Singhal.