An ammonia leak which killed 15 people at a food company in China's commercial hub of Shanghai was caused by a detached pipe cap, state media said Sunday.

The leak from a refrigeration unit at Shanghai Weng's Cold Storage Industrial Co on Saturday injured another 25 who are in stable condition in hospital, Xinhua news agency reported.

It said an initial investigation showed that a cap on a pipe carrying the liquid ammonia had fallen off.

Environmental monitoring vehicles had been sent to assess the impact of the leak, but no pollution was detected as of Sunday, it quoted a statement from the Shanghai Environmental Protection Bureau as saying.

More than 200 firefighters were deployed to the scene, using water to disperse the liquid ammonia and its fumes, the Xinmin Evening News reported earlier.

Liquid ammonia used in refrigeration can destroy lung tissue, potentially resulting in death, and cause chemical burns on the skin, according to the US Occupational Safety and Health Administration.

The food company involved is engaged in the import and export, storage, processing and sales of seafood.

It has the capacity to freeze 150 tonnes of products a day, according to its website.

Shanghai is considered to be more tightly regulated than other cities in China, but this marks the second accident at a food company there in less than a month.

On August 20, five workers at a local food company died after inhaling poisonous fumes while cleaning a vat used for making pickled vegetables, according to the government.

China has a dismal record on industrial safety as lax enforcement of laws causes business owners to cut corners or offer bribes to evade standards.

More than 27,700 people were killed or went missing in workplace accidents in the first half of this year, state media have reported.

A fire caused by an ammonia leak at a poultry processing plant in the northeast province of Jilin in June killed more than a hundred people.