"Some 300 cubic metres of effluent containing sodium cyanide was accidentally discharged" into a drainage ditch at the Borsa mine and then flowed into the Viseu river, a tributary of the Tisza, killing fish in a limited area, the ministry said.

Hungarian and Ukrainian authorities had been informed despite no signs of cyanide being found in the Tisza, a tributary of the Danube, the ministry said.

The Borsa mine was responsible for a major toxic discharge in 2000 when a tailings dam containing heavy metals burst releasing 20,000 tonnes of toxic mud into the Tisza river.