A clean-up operation is under way on an artificial lake in Serbia that activists say has collected more than 20,000 cubic metres of plastics from unregulated dumps.

Environmental activists say that the river Lim, which fills the Potpecko lake, is swollen by melting snows and has carried in more than 20,000 cubic metres of plastics from unregulated dumps along its banks in Serbia, Montenegro and Bosnia.

As the garbage almost clogged a dam on the lake and threatened the functioning of a hydroelectric plant, authorities in Belgrade ordered an immediate clean-up.

Two barge operators collect up to 100 cubic metres of plastic and other rubbish daily and take it to a landfill about 80 km away.

"This problem is years old and it is difficult to say who is responsible, ... from its source, the Lim river ...  flows through three states," Dragan Ljeljen, a official from the nearby town of Priboj, told Reuters.

To counter the problem, Serbia's Environment Minister, Irena Vujovic, and officials from Montenegro and Bosnia agreed on Monday to coordinate efforts to prevent further pollution.

In Priboj, environmental activist Slavisa Lakovic said the persistent pollution is harming wildlife and fish in the lake.

"Piling up trash affects the quality of water and reduces the quantity of oxygen in it ... animals are coming to the banks to rummage through trash," he told Reuters.

Countries in the western Balkans are still recovering from the wars and economic turmoil of the 1990s.

They have done little to tackle environmental issues, in part due to lack of funds.

Serbia, a European Union membership candidate, needs to spend billion of euros to clear up pollution before it joins the bloc, authorities in Brussels and Belgrade say.