About 475 girl students and several teachers were on Saturday admitted to various hospitals in the national capital after a "mildly toxic" gas spread out of a chemical spill from a container stored at a customs depot adjoining two schools in Southeast Delhi's Tughlaqabad area.

Most of the students were discharged after a few hours while some of them were kept under observation in the Intensive Care Unit (ICU) of a few hospitals.

The affected students of the Delhi government-run Rani Jhansi Sarvodaya Kanya Vidyalaya School and Government Girls Senior Secondary School were rushed to nearby hospitals ~ including Batra hospital, Hamdard Institute hospital, Apollo hospital, Safdarjung hospital and ESI hospital (Okhla) ~ as they complained of irritation in eyes, breathlessness and nausea following the chemical leakage in the morning.

However, by late evening, 406 students were discharged from hospitals as their condition was said to be "stable".

The Delhi government has ordered a magisterial probe into the incident. Delhi Police has registered an FIR under the Indian Penal Code and the Environment (Protection) Act.

The fumes, which affected the students of the two schools at Tughlaqabad's Railway Colony, spread from a container containing "2-chloro-5 chloromethylpyridine", which is used in insecticides and pesticides. The chemical was imported from China and was to be taken to Sonepat in Haryana.

The police said the incident was reported at around 7.43 am. The teams of police and National Disaster Response Force (NDRF) as well as CATS ambulance then reached the spot.

The Deputy Commissioner of Police (Southeast) Romil Baniya said after the leakage of a  chemical from Tughlaqabad customs depot area about 475 students of the two girls' schools were taken to five hospitals. About 406 students were later discharged, he said.

The Rani Jhansi School's vice principal Renu Rampal told The Statesman that following their morning assembly the children started complaining about irritation in eyes and breathlessness. "We made calls to police as well as the hospital about the incident after which the girls were rushed to hospital," she said.

An NDRF spokesman Krishan Kumar said a container truck carrying 80 cans of 20 litre capacity, each containing the chemical, left Tuglaqabad container depot for Sonepat at 3.30 am Saturday morning.

"After coming out of depot the driver parked it on the road side near Railway colony Tuglakabad to have tea. During this halt some chemical spilled on the side of the road. After having tea, the driver left for Sonepat. Due to spillage of the chemical, which had started to vaporise, children of adjoining Rani Jhansi school got affected," Kumar said. The NDRF personnel sanitised the affected area.

"A case under sections of 278 (making atmosphere noxious to health), 284 (negligent conduct with respect to poisonous substance), 336 (act of endangering life or personal safety of others) and 337 (causing hurt by act endangering life or personal safety of others) of IPC has been registered against unknown person," Baniya said.

"The students came with complaints of irritation in eyes, breathlessness and pain in abdomen. All of them were admitted and put on conservative treatment,” said officials of Hamdard Institute hospital, where around 250 children were said to have been admitted. “While majority of them have been discharged, two are admitted in the ICU. They are stable,” its superintendent Dr Ajaz Mustafa said.

Delhi Chief Minister Arvind Kejriwal reportedly said, "We have asked the DM to investigate the matter and find out how the gas was leaked at the depot. We have asked the DM to take strict action against those found guilty of negligence.”

Kejriwal's deputy Manish Sisodia, who also handles the education portfolio, spoke with the Union Health Minister J P Nadda about the incident. Sisodia said a team from the AIIMS was carrying out a study on the medical impact of the gas leak.

Nadda asked all Centre-run hospitals to be ready to help the victims. He also constituted a five-member team of doctors to take stock of the situation.

The Delhi administration reportedly issued a show cause notice to the Customs Department and the State-run Container Corporation Of India Ltd (CONCOR), the custodian of the depot, asking it why the depot should not be shut down for storing hazardous chemicals near schools.

"Tughlakabad area is safe now. An NDRF team also visited the consignee factory at Sonepat… All containers have been emptied and washed,” NDRF Director General RK Pachnanda reportedly said.