Profile: Gen Petraeu

General David Petraeus, the outgoing US military commander in Iraq credited for improving security there, has passed control to Lt Gen Raymond Odierno.

The US Defence Secretary, Robert Gates, presided over the ceremony in Baghdad.

Gen Petraeus, who assumed command in February 2007, implemented the "surge" plan, which is credited with reducing the level of violence dramatically.

In his next job leading the US Central Command, he will oversee operations in the Middle East and Afghanistan.

He will take up the post in late October, working from headquarters in Tampa, Florida.

Achievements

Gen Petraeus handed the command of the 146,000-strong US force at a ceremony in one of Saddam Hussein's former palaces on the outskirts of the capital.

Profile: Gen Odierno Sadr City comes back to life

Levels of violence were high across Iraq when Gen Petraeus came to the post in February 2007 - in Baghdad alone, there were 40 car bombings that month.

Mr Gates recalled the challenges that Gen Petraeus had faced on arrival in Baghdad.

"Darkness had descended on this land," Mr Gates said. "Merchants of chaos were gaining strength. Death was commonplace."

He lauded Gen Petraeus and Gen Odierno for their achievements in Iraq, where Gen Odierno served as deputy US commander in 2007.

"Slowly, but inexorably, the tide began to turn," Mr Gates said. "Our enemies took a fearsome beating they will not soon forget."

'Long struggle'

Under the surge plan - which saw nearly 30,000 US troops deployed to trouble spots in Iraq - coalition forces moved out of large bases and into highly populated areas.

Since then, the security situation has improved markedly, with less violence and fewer deaths, and progress on both the political and economic fronts.

The BBC's Mike Sergeant in Baghdad says two other factors were crucial: a ceasefire by the Shia militia and deals with former Sunni insurgents.

But on the eve of Gen Petraeus's departure, a female suicide bomber blew herself up in Diyala province, killing 22 people - a reminder that violence could easily escalate again.

The big challenge for Gen Odierno will be finding ways to stop that sort of violence escalating at a time when the number of US troops are shrinking, our correspondent says.

In a BBC interview before his departure, Gen Petraeus said he would never declare victory in Iraq.

He said the recent security gains were "not irreversible" and that the US still faced a "long struggle" in the country.

When asked if US troops could withdraw from Iraqi cities by the middle of next year, he said that would be "doable".

Last week, Mr Bush announced a cut of 8,000 US troops in Iraq by February - with some 4,500 being sent to Afghanistan.