By Subir Bhaumik
BBC News, Calcutta
Thousands of dead fish have been washed ashore in a river in the north-eastern Indian state of Assam river since early this week.
Pollution may be responsible (Pics: Subhamoy Bhattacharjee)
They were discovered on the banks of the Brahmaputra river in the state capital, Guwahati.
An equal number of sick fish were found in the same place, officials said. They blamed rising pollution levels.
An investigation has been ordered by the Assam government, said C.K.Bhuiyan, senior district official in Guwahati.
Earlier in the year, the Assam Pollution Control Board (APCB) told all the oil refineries in the state to achieve "zero pollution" levels by 31 December, otherwise they would be threatened with closure.
The cause of the fish deaths is being investigated
The Guwahati refinery was found to be the worst polluting refinery and was perhaps the only one in Assam not to have yet complied, pollution control board officials said.
"The marine life in the Brahmaputra river has been seriously affected by the pollution caused by these refineries, particularly the Guwahati refinery," said Jawaharlal Dutta, APCB chairman.
He alleged that pollution from these refineries was several hundred times above the permissible limits.
But district officials who are monitoring the development were not ruling out other possible causes.
"It could be a case of poisoning caused by water pollution or may be an outcome of some kind of explosion inside the water to catch fish in large numbers from the river. We are not ruling out either possibility at this moment," Mr Bhuiyan said.
Killing of fish by using explosives and chemical fertilisers or other poisonous substances is not uncommon in Assam, especially during the winter.