Union Home Minister Rajnath Singh said the leak has been "plugged" and a team from the atomic energy department is on the spot.
According to Customs officials, cargo operations have been suspended till safety clearance is given by the concerned agencies.
"Entire area has been cordoned off. Staff have been evacuated. All cargo operations have been suspended till concerned agencies give safety clearance," Chief Commissioner of Delhi Customs B K Bansal told PTI.
The matter came to light when a pair of loaders felt irritation while handling containers that arrived on board a Turkish Airlines craft at 4:35 AM. Leakage was observed from four of the 10 containers of 13 kg each that were imported from Turkey.
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The National Disaster Response Force (NDRF) was called in to check the substance and contain and plug the leak, sources said.
The material, said to be low radio activity Sodium Iodide Liquid Class 7, was imported for medical purpose by a Delhi-based multi-speciality hospital.
Meanwhile, aviation regulator DGCA has launched a probe into the incident.
A team headed by the Director of Air Safety has been constituted to conduct an investigation, the Civil Aviation Ministry said in a statement.