The DaimlerChrysler vehicles will arrive in China in September, where they will join the Beijing Public Transport Corporation's 17,000-strong bus fleet.

"(They) will enter regular service on a 19-km long route...which is extremely busy and passes the world-famous Summer Palace as well as the site for the next Olympic Games," the firm said in a statement on its website.

Fuel-cell engines run on hydrogen and emit only water vapour.

The project, which will last until October 2007, will test the practicality of the technology on Beijing's sometimes chaotic roads and the response from the city's residents.

The hydrogen will be supplied by oil major BP , DaimlerChrysler said.

Other projects mooted to help the city clear away the grey haze that often hangs over it range from an exhaust-linked car tax to rooftop lawns.

China's capital said it had just managed to reach its goal of 227 days with clean air in 2004 after the target seemed all but unattainable last October. Sceptics have raised questions about the standards used to make the claim.