A total of 10 kg (22 lbs) of ammonia leaked on Monday during a scheduled defrosting of a refrigerating chamber due to safety violations and operator error.

"A total of 12 people sought medical help, seven of them were rushed to the city's hospitals with respiratory poisoning," the source said.

Industrial accidents with ammonia are frequent at Russian plants. Last September, one person died after a pipe ruptured causing ammonia gas to leak into a dairy plant in Russia's second largest city of St. Petersburg. Several months before, two people died from ammonia poisoning after an explosion at a meat plant in south Russia's Saratov Region.