A firefighter leaves the accident site at the Shanghai Wengpai Cold Storage Industrial Co Ltd in the Baoshan District of Shanghai, Aug 31, 2013. [Photo/Xinhua]

A liquid ammonia leak from a food company's refrigeration unit in suburban Shanghai killed at least 15 people on Saturday, local authorities said.

The leak had also made 25 others ill and seriously injured five as of press time on Saturday. All those affected were sent to the hospital, the Shanghai Administration of Work Safety said.

Emergency workers said the accident was caused by a pipeline leak at a workshop of Shanghai Wengpai Cold Storage Industrial Co, a private enterprise engaged in aquatic products' import and export, storage, processing and sale.

Set up in 2006, Wengpai has the capacity to freeze 150 metric tons of products a day and employs about 200 workers, its website said.

Authorities have set up an investigation team and promised to publicly disclose the results.

The accident took place at about 10:50 am on Saturday in the city's northern district of Baoshan. More than 200 emergency workers arrived at the scene in 25 fire trucks. They evacuated the area, diluted the chemicals with water and searched for survivors.

Hospital, public security and environmental protection bureaus' workers also rushed to the site as police cordoned off the area.

Ammonia's stench was still strong in the early afternoon. Employees were allowed to return to the factory at about 3 pm, after environmental monitors determined the area to be safe.

The city's environmental protection department said the leak has not affected nearby locations.

"Baoshan district's environmental monitoring station has been monitoring downwind areas," the city government said in a statement.

"It has not found the accident has affected surrounding areas."

Authorities also ruled out the connection between the leak and the pungent odor that turned up at about 8 am on Saturday on the border of Baoshan and Zhabei districts.

Preliminary analysis showed the odor was caused by an unspecified government department staging a safety drill involving smoke flares, the government said.

Liquid ammonia is pressurized in meat processing plants' cooling systems and can cause fatal lung tissue destruction and chemical burns to the skin.

On June 3, an inferno caused by an electrical short circuit led to the leakage and explosion of ammonia, killing 121 at a slaughterhouse in Jilin province.

That fire broke out at 6:06 am at a poultry processing workshop owned by the Jilin Baoyuanfeng Poultry Co in Dehui city's Mishazi township, about 100 kilometers northeast of the provincial capital Changchun.

wangzhenghua@chinadaily.com.cn