The Air Traffic Control (ATC) had given clearance to both the aircraft, according to Indigo airline spokesperson.
The Air India flight 879, carrying 120 passengers, was descending and the Indigo Bagdogra-Delhi flight 6E472 with 130 passengers on board was given clearance to climb the 30,000-feet height, Indigo spokesman said.
The Indigo flight departed at 1222 hrs. As they came close to each other violating the 1000m separation, the pilots of the Indigo plane and the Air India aircraft, however, manoeuvred themselves to avoid a near-miss, the spokesperson said.
The Indigo flight Captain got a traffic collision avoidance system (TCAS) resolution advisory (RA). As per Standard Operation Procedure (SOP), the Capt followed RA and descended and the AI flight also followed RA and turned right, the spokesperson said.
Once the Captains received "clear-of-conflict" message, they manoeuvred back to normal situation, he added.
The Indigo flight landed at Delhi at 1415hrs. The DCGA is conducting an inquiry into the incident.