BEIJING -- China's State Oceanic Administration (SOA) is widening its scope and sampling water at more sites despite falling densities of key toxic substances in waters near the Tianjin blast site.

Minute traces of cyanide were detected in two out of 13 sites Sunday at a maximum density of 0.00156 mg/L. One out of 16 sites reported a density of 0.00103 mg/L the following day. On Tuesday, no traces were found, the SOA said Wednesday in a statement.

The maximum density detected was below the 0.005 mg/L standard for first-grade water quality, which is fit for the oceanic fishing industry and protected habitats for rare species.

The monitoring sites have been set up off Tianjin port and the sea east of Dongjiang port, both in the municipality.

Similarly, maximum density of volatile phenol, which was detected in four sites Monday, dropped from 0.00581 mg/L to 0.00477 mg/L Tuesday. First-grade water quality requires no higher than 0.005 mg/L.

Despite the highest density exceeding permitted standard once, the SOA noted that volatile phenol is unlikely to affect nearby waters, but stressed that cyanide remained the most severe threat to the maritime environment, capable of killing large numbers of fish and other marine life.

The SOA is keeping closer track of cyanide dissemination with a wider monitoring scope and more sampling sites, promising to prevent the toxic substance from entering the ocean in significant amounts.

Two explosions ripped through a warehouse storing hundreds of tonnes of toxic chemicals, including roughly 700 tonnes of sodium cyanide, last Wednesday night. The death toll from the accident has risen to 114, with 692 people still in hospital and 57 unaccounted for.