Washington  - President Barack Obama's administration on Tuesday for the first time proposed limiting the greenhouse-gas emissions of cars in the United States in a bid to curb pollution that is blamed for global warming.

The new rule would raise fuel-economy standards and place new requirements on emissions beginning in 2012. The proposal is open for public comment for 60 days before being adopted.

"It's an action that is long overdue," Obama said during a speech at a General Motors plant in Ohio. "It will give our auto companies clarity and stability and predictability."

Carbon dioxide from cars accounts for about one-quarter of all US greenhouse-gas emissions. Vehicles would be required to emit 250 grams of carbon dioxide per mile by 2016 in a bid to reduce auto pollution about 21 per cent by 2030. The measure would be the equivalent of taking 42 million cars off the road.

The proposal would also raise fuel-efficiency requirements to an average of 35.5 miles per gallon (6.63 litres per 100 kilometres) by 2016. Fuel economy is currently about
25 miles per gallon.

US carmakers have been under pressure to focus more on smaller, greener cars like those offered by many foreign competitors. Flagging sales in the midst of a recession drove General Motors and Chrysler into bankruptcy earlier this year, from which they emerged part-owned by the government.

Transportation Secretary Ray LaHood said the new emissions and fuel-economy proposals would "bring about a new era in automotive history."  dpa