Authorities in Shanghai have been offering emergency water supplies to residents of a southwestern suburb after a discharge of chemical waste into a river required the water to be cut off to about 30,000 people.

Firefighting departments were organized to deliver clean water in fire engines for residents living along the lower reaches of the contaminated river, and three large supermarkets and bottled water suppliers have been asked to shift more products to the affected districts — Jinshan and Songjiang.

Police said they had detained 10 suspects from a local logistics company linked to the illegal discharge of pollutants containing styrene — a chemical hazardous to the intestines, kidneys, and respiratory system if ingested — from a tanker into the river in Jinshan.

Urban areas of Shanghai have remained unaffected so far, said the authorities, but it is estimated it will take two days to resume normal water supplies in the affected areas.

The incident, coming shortly after the major chemical contamination of a river in North China's Shanxi province, has highlighted the country's toughening battle to ensure residents have access to safe drinking water.

Residents in the two districts affected started smelling a strong odor at around 7:40 pm on Thursday with some reporting dizziness or other discomfort.

Inspectors later found that the source of the pollutants came from the river in Jinshan, which contained high levels of styrene. Water supplies from two plants in Maogang township in Songjiang were cut off early on Friday morning as a result, and water plants in nearby regions were also watched closely.

"We have mobilized 14 fire engines to dispatch fresh water to eight villages in the township," said Wu Jianliang, the head of Maogang, according to a report on eastday.com.

"Supermarkets and small shops were also contacted to deliver 5 liters of clean water to each household to ensure residents have access to drinking water," he added.

Bottled water at local supermarkets was still sold out on Friday, with residents standing in long lines to fetch water dispatched from fire engines.

Authorities in nearby neighborhoods were also ready to offer clean drinking water, once their water sources had been tested.

"We have launched an emergency plan and are standing by," said Bao Linjun, an official with the Lindong neighborhood committee in Jinshan district.

"We have notified Hualian supermarkets to offer bottled water if required. But currently the water source remains unaffected in our region."

Authorities, meanwhile, have intensified their inspections at rivers in Songjiang, Minhang, Jianshan and Qingpu districts, taking tests every hour.

The environmental protection department said the air quality has improved, though the pollutants had drifted to Jiashan and Pinghu in nearby Zhejiang province by early Friday.

Police said they are still investigating the incident.