Petraeus, the general credited with pulling Iraq back from all-out civil war, handed over control of the 146,000-strong force at a ceremony at a former Saddam Hussein-era palace turned US base near Baghdad airport.

But Odierno, a towering four-star general, said he was aware of the tough task ahead despite a dramatic fall in violence to four-year lows attributed to a "surge" strategy.

"Iraq is now a different country from the one I had seen first. However, we must realise that these gains are fragile and reversible," he said.

On the eve of the transfer, Odierno was given a powerful reminder of the violence when a series of bomb blasts killed at least 34 people.

"He knows we are at a pivotal moment -- where progress remains fragile and caution should be the order of the day," US Defence Secretary Robert Gates said at the ceremony attended also by top Iraqi officials.

"And as we proceed further into the endgame here, I am sure he will make tough but necessary decisions to protect our national interest."

Odierno, Petraeus's former deputy, said the authorities in Baghad will have to shoulder more responsibility for security.

"Time is changing. Iraq is going to take more responsibility. The environment has changed. What I hope to see is a change from military to political, economic and diplomatic strategy," he told reporters.

Petraeus becomes the new chief of Central Command in charge of the US military's biggest challenges -- Iraq, an expanding campaign in Afghanistan, an Al-Qaeda revival in Pakistan and sabre-rattling by Iran.

He said Odierno was the "perfect man for the job."

On Wednesday, Petraeus will meet US President George W. Bush and discuss his new duties.

"It's an appropriate time to meet with him as he takes on the challenges of the broader Middle East," Gordon Johndroe, White House spokesman, said.

Iraq was spiralling into all-out civil war when Petraeus, 55, took charge in February 2007, four years after Saddam was toppled by US invading forces.

But since late 2007 violence has fallen, and much of the credit has gone to the counter-insurgency strategies of Petraeus, whom Gates said had taken command when "darkness had descended on this land."

"Merchants of chaos were gaining strength. Death was commonplace. Around the world, questions mounted about whether a new strategy -- or any strategy, for that matter -- could make a real difference."

"You have dealt enemies of the United States and Iraq a tremendous, if not mortal blow. History will regard you as one of our nation's great battle captains."

Petraeus oversaw the surge, but it was Odierno who first proposed it in December 2006 to a resistant Pentagon, setting the stage for what would become a pivotal turn in the unpopular war.

Odierno, a hulking artillery man criticised for running roughshod over civilians during his first tour to Iraq in 2003-2004, implemented the "surge" as corps commander from December 2006 to March 2008.

He carried out the detailed campaign that poured US troops into Baghdad, cleared Al-Qaeda insurgents from havens surrounding the capital, and targeted Shiite extremists.

Gates said the current mission was in "transition" phase as troop numbers shrink and more provinces are handed to Iraqi control.

"The challenge for General Odierno is how do we work with the Iraqis to preserve the gains that have already been achieved, and expand upon them even as the number of US forces are shrinking," Gates said.

Iraq has taken over handling of security in 11 of its 18 provinces and plans to add a couple more by end of 2008.

Odierno has argued consistently against sharp cutbacks in force levels in Iraq, which is negotiating a security pact with Washington to determine troop levels after a UN mandate expires at the end of the year.

The US force will shrink by about 8,000 troops by January, when Bush leaves office, and pressure for further reductions is intensifying as attention shifts to Afghanistan.

Odierno said this cut "will be automatically replaced by Iraqi forces," adding further reductions would be in consultation with Baghdad.

He said Iraq's delayed provincial election and next year's national election were "milestones" that would determine the nation's long term viability.

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