The level of toxic cyanide in water samples taken from a massive crater created by the Tianjin chemical blasts exceeded safety limits by over 800 times, authorities said Thursday.

The crater, about 60 metres wide and 7 metres deep, appeared after strong explosions and subsequent fires tore apart a logistics centre about 145km southeast of capital Beijing. It has been filled with water following rainy weather in the past week.

Blast crater in the port of Tianjin. Photo: EPA.

The local government said cyanide contamination in the crater is not evenly spread as some samples only tested levels exceeding safety standard by two times, according to Chinese new media The Paper.

Authorities have set up 50 samplings points in the crater and conducted three tests so far, The Paper said.

It will take two to three months for the government to finishing treating the 50,000 tonnes of contaminated water, said Li Xingchun, a member of an expert panel tasked with examining the environmental impact of the blasts.

Meanwhile, thousands of dead fish have appeared in the Haihe River, which runs through Tianjin. The government said Thursday night the fish was not killed by cyanide poisoning.