Eleven people have died and four others are in hospital after a gas leak in northern India.

The incident occurred at an industrial area in Ludhiana city, Punjab state, but the source of the gas leak is still unclear, police told the Press Trust of India (PTI) news agency.

The National Disaster Response Force has sealed off the densely populated area and evacuated residents.

Authorities are trying to ascertain the type of gas in the leak, PTI quoted officials as saying.

Ludhiana deputy commissioner Surabhi Malik told the news agency it was possible that the gas may have spread from manholes.

“We are going to collect samples from manholes. It is quite likely that some chemical reacted with methane in manholes,” she said.

The state’s chief minister, Bhagwant Mann, said the leak came from a factory and added on Twitter: “All possible help is being provided.”