"GM is pushing very hard on fuel cells," Fronk told a conference in London. "We need to solve issues in performance, cost and durability," he said.

"We have to see all these (solved) before we can pull the trigger and start commercial production," Fronk said.

Fuel cells use a chemical reaction to produce electricity from hydrogen. The process emits only tiny amounts of carbon-dioxide, the main greenhouse gas blamed for global warming.

President George W. Bush has backed the use of hydrogen-fuelled cars as part of a strategy to tackle climate change by developing new energy technologies that curb the world's dependency on oil and gas.

Commercial applications of fuel cells in laptop computers and mobile telephones could emerge within a few years, analysts say. But putting fuel cells into cars poses greater challenges.

"We have noticed and taken care of a lot of problems in design but we are still doing a lot of learning," said Fronk. "Each of the car companies is taking a slightly different approach."

Fuel cell designs with long enough life spans and the ability to power cars over long enough distances before refuelling will need to emerge before the new breed of hydrogen cars hits the showrooms.

Storage and distribution of hydrogen raises big issues. "The number one barrier (to commercial use of fuel cells) is hydrogen storage," said Valri Lightner, fuel cell team leader at the US Department of Energy.

The DoE estimates the cost of producing power from fuel cells needs to drop to $30 per kilowatt from a current cost of about $110, based on the production of half a million units per year.