A call was received at 6.13 pm about the incident, following which 12 fire tenders were rushed to the spot, Chief Fire Officer Atul Garg said.
"The fire is completely under control now. There are no casualties. The upper floor was vacated due to smoke," Garg told the media.
The fire started from the basement of the building where an Operation Theater is located, the fire department said, adding that the patients on the ground and first floor were evacuated. "The leakage of oxygen from a pipe is being fixed."
The blaze also affected services at the Trauma Centre, which caters to patients from all across the National Capital Region, apart from the city.
"The fire damaged several of the surgical equipments in the operation theatre but there was no report of any casualty as the patients and doctors were evacuated on time. The fire is under control but the operation (to douse it) is still on," IANS quoted a fire official as saying.
The cause of the fire could be due to a short-circuit, he said.
A team of the National Disaster Response Force (NDRF), whose office is located nearby, helped to shift the patients to other parts of the hospital, the official added.
Hospital sources said patients had to be shifted to another ward and others were evacuated since the smoke engulfed the ground floor of the building.
The electricity supply has been stopped in the building for now and short circuit is suspected to be the reason behind the incident, they said.
The AIIMS administration has instituted an inquiry into the matter to identify the cause of fire and to suggest further measures for improvement.
(With nputs from agencies)