ULAANBAATAR, April 11 -- Mongolia's love affair with the Toyota Prius is turning into an environmental headache as thousands of ageing hybrid batteries pile up with nowhere to go.

"If you throw a rock in Mongolia, chances are you'll hit a Prius," a Mongolian taxi driver told Singapore-based news outlet CNA in a recent report.

Cheap, second-hand Priuses imported from Japan now dominate roads from the capital to the remote steppe, prized for their durability and fuel savings.

Hybrid vehicles make up about 45 per cent of the country's 1.5 million vehicles, with most arriving already heavily used.

Many come with batteries nearing the end of their lifespan, which fail even faster in Mongolia's extreme cold and rough terrain.

When they do,...