"Breaking our oil addiction is one of the greatest challenges our generation will ever face. It will take nothing less than a complete transformation of our economy," he said.

Fuel prices in the US have rocketed in recent months as the global cost of crude oil rose to well above $140 a barrel.

Second reversal

Obama laid out his energy plan in Michigan, the state that is home to struggling US car makers like Ford, and also a key battleground state in November's election.

Part of Obama's plan also calls for a cut in consumption, so that the US can save more oil than it currently imports from the Middle East and Venezuela.

Obama's campaign also said he would create five million new jobs by investing in clean energy schemes.

His reversal on the use of the emergency oil stockpile is his second shift on energy issues in recent days.

On Friday, he dropped his blanket opposition to offshore oil drilling and signaled he would be open to limited drilling as part of a compromise package aimed at lowering prices.

Attack advertisement

Obama also used the energy issue to attack his rival, John McCain, the Republican candidate.

In a new campaign advertisement, he attacked McCain, claiming he had taken campaign contributions from big oil firms.

In focus

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"Now big oil's filling John McCain's campaign with two million dollars in contributions," the advertisement said.

The spot also linked McCain to George Bush, the US president, whose popularity ratings remain low.

"After one president in the pocket of big oil ... we can't afford another," the ad said.

Tucker Bounds, a McCain spokesman, condemned the Obama advertisement as "hypocrisy".

"Barack Obama's latest attack ads shows his celebrity is matched only by his hypocrisy,'' Bounds said.

"After all it was Senator Obama, not John McCain, who voted for the Bush-Cheney energy bill that was a sweetheart deal for oil companies.

"Also not mentioned is the $400,000 from big oil contributors that Barack Obama has already pocketed inthis election."