Eleven people, including three children, died allegedly after inhaling toxic gas in the city's thickly-populated Giaspura area on Sunday, with authorities suspecting that dumping of some chemicals in the sewer led to the noxious emission.

Four more persons, who were taken ill, are undergoing treatment at a hospital, officials said, adding the area was sealed, the residents were evacuated and a National Disaster Response Force (NDRF) team was at the spot following the incident.

Though the source of the leakage and the type of the gas were yet to be ascertained, the officials said that they suspected that the poisonous gas was released after some chemical was disposed of in the sewerage in the area and reacted with methane.

Giaspura is a thickly populated area with a migrant population. Several industrial and residential buildings are located here. All the victims belonged to Uttar Pradesh and Bihar and had been staying in Ludhiana.

The incident came to light Sunday morning when some people who came to a local grocery store to buy milk started fainting. Four persons died on the spot while others were rushed to the hospital. Among the dead were three members of the family that owned the store and five from another.

Police said five females and six males were among the dead.

The district administration announced Rs 2 lakh compensation each for the family of the deceased and Rs 50,000 each for those who were taken ill in the incident.

Union Home Minister Amit Shah expressed grief at the loss of lives and said the NDRF team is engaged in relief work. Punjab Chief Minister Bhagwant Mann Bhagwant Mann said the incident was very painful and added that all possible help is being provided.

“It is likely that there is some gas contamination. We are collecting samples from manholes. It is quite likely that some chemical reacted with methane in manholes," Ludhiana Deputy Commissioner Surabhi Malik told reporters.

As the smell of the gas started spreading, many people moved out of the area.

The DC said the cordon at the scene of the leak will be extended. She said those who died in the incident did not show any symptoms of respiratory problems.

"It is likely that because of neurotoxins the deaths took place," Malik added.

Additional Deputy Commissioner Amarjit Singh Bains said, "A broken manhole has been found in the area and there was a strong smell emanating from it. There is a possibility that the chemical has been poured into the sewerage from that point".

A district administration official said a team of the National Disaster Response Force (NDRF), which has reached here, will ascertain the source and the type of gas.

The Ludhiana DC said an NDRF team is engaged in extracting samples for various kinds of chemical tests.

She said there was no need to panic and asked people not to fall prey to any rumour-mongering.

She also appealed to people to stay away from the area. "A check will be conducted to see how far this gas has spread," she said.

"We are collecting samples from manholes to just check what it is exactly and to what extent it spread," Malik added.

She further said that CCTV cameras were being scanned.

The DC said different teams of district administration, police, municipal corporation and Punjab Pollution Control Board are stationed at the site.

Ludhiana Police Commissioner Mandeep Singh Sidhu said it was apprehended that someone disposed of some chemicals in the sewerage which led to the emission of the toxic gas.

"A cat was also found dead at the incident site," said Sidhu.

Through the public address system, people are being called to stay away from the area, said Sidhu.

Police also deployed drones to check for other victims who could be lying unconscious.

The deceased were identified as Abhay (13), Aryan (10), Kalpna (16), Kamlesh (60), Varsha (35), Sourav (35), Preety (31), Kavilash (40), Navneet (39), Neetu Devi (39) and an unidentified person.

Heart-rending scenes were witnessed outside the mortuary of the civil hospital, where the bodies were brought for post-mortem.

"The incident of gas leak in Giaspura area of Ludhiana is very painful. Teams of police, district administration and NDRF are present on the spot. Every possible help is being provided...," Chief Minister Mann said in a tweet in Punjabi.