An Army general who oversaw the US withdrawal from Iraq assumed command in the Middle East Friday, succeeding an officer who had clashed with the White House over handling tensions with Iran.

General Lloyd Austin, 59, who will oversee the pullout of US troops from Afghanistan in 2014, took the reins of the military's powerful Central Command in a ceremony in Tampa, Florida, succeeding General James Mattis, a blunt-speaking Marine who served in three wars.

US officials acknowledge that Mattis, a four-star general, had disagreed with White House advisers over policy towards Iran, favoring a more robust approach to Tehran's controversial nuclear program.

The friction with the White House prompted speculation that Mattis may have been forced to cut short the customary three-year tenure for the CENTCOM post by several months.

But Pentagon officials denied there was any attempt to push him out early.

Mattis was lavished with praise at the change of command ceremony at MacDill Air Force base, with Defense Secretary Chuck Hagel calling him a "legendary figure" who was beloved by his troops.

"Jim Mattis has been front-and-center in every major combat operation this nation has conducted for more than two decades," said Hagel, recounting how Mattis led troops in the 1990-91 Gulf War, in Afghanistan after the September 11 attacks and in the 2003 invasion of Iraq.

The next chief of US Central Command, which covers 20 countries from the Middle East to Central Asia, will have to grapple with the drawdown in Afghanistan through 2014, tensions with Iran and Syria's civil war.

Without mentioning Iran, Austin vowed that the United States would back its allies and defend regional security.

"The US will continue to play an important role as a key partner to our friends and our allies," Austin said in a speech.

"We'll provide them with the necessary support and we will stand ready and willing to hold accountable those who have threatened regional security and stability through their actions or through the action of proxies," he said.

Mattis has earned a reputation for colorful comments during his career, but kept a low public profile during most of his tenure at Central Command.

In his farewell remarks on Friday, Mattis spoke of his admiration for the troops.

"I would happily storm hell in the company of these troops, who I haven't the words sufficient to praise, so I will not try."