Thousands of dead fish have been spotted on the riverbank of a Tianjin river, several kilometres from a deadly blast in an industrial complex, a day after authorities had declared the city's drinking water was safe, state media reported.

State-run Xinhua news agency said analysis of water samples taken from a section of the Haihe River found no toxic levels of cyanide.

Tianjin officials said the dead fish were caused by regular seasonal low oxygen levels in the water and were not related to the blasts.

State authorities have confirmed that more than 700 tonnes of the deadly chemical sodium cyanide were stored at the Tianjin warehouse that blew up and killed 114 people last week.

Authorities have also warned that cyanide levels in waters around Tianjin port, the world's 10th-busiest and the gateway to China's industrial north, had risen to as much as 277 times acceptable levels.