The warehouse explosion site in Tianjin, North China, Aug 14, 2015. [Photo/Xinhua]

Investigators searched for clues on Friday to identify what caused two huge explosions at a warehouse storing volatile chemicals at a busy port in north China, as foreign and local companies assessed the damage to their operations.

The dangerous chemicals stored in the warehouses that exploded on Wednesday night in Tianjin Port can not be determined at the moment, authorities said at a press conference on Friday.

Gao Huaiyou, deputy director of Tianjin's work safety watchdog, cited major discrepancies between the accounts of company management and customs, and damage to the company's office as reasons they are unable to identify the chemicals.

Cargo is stored in a warehouse for no more than 40 days before being transferred elsewhere, Gao said, adding that the blast sites have been redesigned to contain dangerous chemicals.

At 11:28 am Friday, a small explosion hit the site again and white smoke billowed. Authorities said at the news conference on Friday that by noon all flames will be extinguished and then containers would be moved to safer places and be opened for examination.

Hazardous substances have been detected from two drainage outlets near the blast site in Binhai New Area, the Ministry of Environmental Protection said on Friday morning, refuting rumors that the dangerous participles have polluted the surrounding area.

The harmful pollutants, including chemical Oxygen Demand (COD) and cyanide - a highly toxic chemical compound, were found three times and eight times higher than the usual levels, according to the ministry.

The Tianjin municipal environmental watchdog has set up 22 monitoring stations, of which five are checking water quality.

The drainage outlet to the Bohai Sea has been closed, and the bureau is conducting an investigation into water quality.