“I can confirm that we have seen sporadic losses of fish in the main branch of the Danube,” regional chief for the disaster relief services Tibor Dobson told AFP.
“The fish have been sighted at the confluence of the Raba with the Danube,” where water samples had shown a pH value of 9.1, he said.
“Fish cannot survive at pH 9.1,” the official added.
The pollution reached the main branch of the Danube at around midday (1000 GMT) and had already destroyed the entire ecosystem of the much smaller Marcal, officials announced earlier.
Water alkalinity is a measure of river contamination and on a scale of 1-14, pH values of 1-6 are acid, between 6 and 8 are neutral, and readings of 8-14 are alkaline.
The toxic spill poured from a reservoir at an alumina plant in Ajka, 160 kilometres west of Budapest, which burst on Monday, sending 1.1 million cubic metres of red sludge into surrounding villages.
Four people died in one of the villages, Kolontar, from where the tiny Torna stream flows into the Marcal.
The Marcal is a tributary of the Raba, which in turn flows into the Danube, that runs from Hungary through Croatia, Serbia, Bulgaria, Romania and Ukraine before reaching the Black Sea.
Romania has asked Hungary for more information on the toxic sludge spill and said it is gearing up for the risk of drinking water contamination in towns along the river.
Adrian Draghici, head of the water management authority in Mehedinti county, 400 kilometres west of Bucharest, said the pollution wave could reach Romania on Saturday, adding that water quality controls have been reinforced.
Romanian authorities complain Hungary has not informed them of the exact nature of the heavy metals contained in the toxic mud.
Environmentalists have expressed alarm about the possible long-term effects of the disaster.
“The heavy metals are the danger in the long run,” Gabor Figeczky, acting head of nature protection body the WWF in Hungary, told AFP.
“Heavy metals such as lead, arsenic or chrome cannot dissolve in alkali, but in acid they can, and experts have used acid to neutralise the important alkaline content of the red mud,” he said.
“If the metals dissolve, they sink to the river bottom and get into plants and fish and that’s a poisoning very difficult to deal with. But we have to wait for the lab results to be able to give estimates.”
Figeczky said that “following such devastation, the Marcal’s ecosystem could take between three and five years to recover.”
Water samples from the Torna showed pH levels of around 10 on Thursday but these were falling, Dobson said. On Monday the level was 13.5.
Groundwater in the wells in the region showed neutral readings of pH 7.5.
A spokesperson for Greenpeace, Denes Szabo, warned that the neutralising efforts were also a danger.
“Our main concern is that now that all these neutralising agents are in the river, they may tip the balance in the other direction, causing even more harm,” he said.
Earlier, Prime Minister Viktor Orban on a visit to Kolontar suggested the destroyed village may have to be rebuilt elsewhere, because the ground has been rendered inhabitable by the sludge.
He insisted Hungary did not need financial help, but would welcome expertise to help clean up the spill.
In Brussels a European Commission spokesperson said the EU was prepared to assist Hungary or any other state affected but had not yet received any request for help.