Officials said they were still trying to determine what had caused the deaths of up to one million caged tubtim fish, a type of tilapia, at different locations along the river about 100 kilometres (60 miles) north of Bangkok.

Jaranthada Karnsasuta, director general of the fisheries department, said a sudden lack of oxygen in the water killed the fish.

"Oxygen in water is very poor. Some reported zero to 0.5 percent of oxygen in the water, while fish need more than three percent to survive," he told AFP.

He said they were currently investigating two possible explanations -- that a sugar boat which capsized earlier this month released toxic byproducts into the river, or that upstream factories had polluted the waterway.

Local villagers and farmers suspect that factories, including one that produces the food additive monosodium glutamate, had released untreated water into the Chao Phraya, which flows down to the capital Bangkok, Jaranthada said.

It was less likely to have been caused by the capsized boat, he said, as the accident happened March 3 and the fish would have been affected earlier.

The Ministry of Agriculture will compensate fish farmers for their losses, which total about 40 million baht (1.3 million dollars), Jaranthada said.

An official in Ang Thong told AFP that the public health ministry had reassured him that the dead fish were not poisonous to humans, but added that they would be buried rather than entering the food chain.

Jaranthada said that the quality of water on the affected stretch of the Chao Phraya was improving after the irrigation department released clean water from an upstream dam.

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