America's big automakers enjoyed bumper sales in August as US consumers snapped up new cars and gave a boost to President Barack Obama's bailout of the industry.

GM sold 240,520 new vehicles last month, its highest figure for the year and 10% higher than in August last year. Chrysler's sales rose 14% compared to August 2011 to 148,472, its best August sales figure since 2007.

Both companies were bailed out by the Obama administration and forced into bankruptcy. That bailout has become a central plank of Obama's re-election campaign and has been attacked by his Republican Mitt Romney as "crony capitalism".

Ford, the only major US car firm not to enter bankruptcy, reported a sales increase of 13% to 197,249 vehicles as drivers bought smaller cars.

The sales figures come as Obama is expected to champion his auto bailout at this week's Democratic convention. GM's results contained another fillip for the president: a new sales record for its controversial Chevy Volt.

The plug-in hybrid electric car has proven a politically charged vehicle with the government accused of meddling in GM's affairs in order to get the car built, charges senior GM officials have denied.

The car been attacked on a number of occasions by Mitt Romney who has called it "an idea whose time has not come". In February Obama said he intended to buy a Volt when he leaves office.

GM sold 2,831 Volts last month, its best sales ever. The sales came as overall sales at Chevrolet jumped 25%.

Michelle Krebs, auto-industry analyst at Edmunds.com said pent-up demand for cars and easier access to credit were driving sales up and were likely to continue to do so.

"The average car on the road is now over 10 years old, that's a record number," she said. "We are on track to sell 14.4m cars this year; there's an awful lot of pent up demand."

She said the figures were "good news for Detroit and good news for Obama. Clearly GM and Chrysler wouldn't have been around if it hadn't been for the bailout".