Serbian and Croatian authorities stepped up testing in the Danube on Friday amid fears the Hungarian toxic spill is spreading.
Officials say there is no sign of any pollution in their waterways but are drawing up emergency plans in case. Romanian waterways are also feared to be under threat.
The Danube runs from Hungary through Croatia, Serbia, Bulgaria, Romania and Ukraine to the Black Sea. The spill happened on Monday at an aluminium plant in Ajka, 160 kilometres west of Budapest
Marine life has been wiped out in the Marcal river, which feeds into the Danube. Crews have been pouring plaster and acetic acid into waterways to try to bring pH levels down.
Some 1.1 million cubic metres of red sludge escaped from a reservoir at the plant and into surrounding villages, killing four people and injuring 100. Farmland and homes have also been flooded and destroyed.
Earlier this week, Hungary opened a criminal investigation into how the disaster happened.
Serbia, Croatia test for Hungarian spill pollution