WASHINGTON (CNN) -- Army Gen. David Petraeus, the top U.S. commander in Iraq, has been chosen to become chief of U.S. Central Command, Defense Secretary Robert Gates said Wednesday.

Gen. David Petraeus has been the commander in Iraq for more than a year.

Petraeus would replace Adm. William Fallon, who said last month that he was resigning. Fallon said widespread, but false, reports that he was at odds with the Bush administration over Iran had made his job impossible.

In addition, Gates said, Army Lt. Gen. Ray Odierno, commander of the Multinational Corps-Iraq -- the No. 2 position in Iraq -- is being nominated to fill Petraeus' post. Odierno has been home from Iraq for only a couple of months but has agreed to return, Gates said.

The plan is for Petraeus to leave Iraq in late summer or early fall, Gates said, to ensure a smooth transition and plenty of time for Odierno to prepare.

"We expect to move the paperwork on these nominations to the White House and to the Senate very quickly," Gates said. "Because of the complexity of this series of moves, I respectfully ask the Senate to move on them expeditiously, hopefully by Memorial Day, so the families and we can plan appropriately."

Odierno was in line for a Pentagon desk job as Army Vice Chief just over a year after helping Petraeus implement the "surge" in U.S. troops in Iraq.

The Pentagon says Petraeus will stay in Iraq long enough to make the first recommendation on further troop cuts after the surge is over. Watch the challenges ahead of Petraeus »

Gates said the generals' promotions reflected an endorsement of the current course in Iraq.

"The course certainly that Gen. Petraeus has set has been a successful course. So, frankly, I think staying that course is not a bad idea. I would say it's a good idea," Gates said. Watch Gates nominate Petraeus »

Central Command, which oversees U.S. forces in East Africa, the Middle East and Central Asia, has its headquarters at MacDill Air Force Base in Tampa, Florida.

Gates said Odierno is well known to troops in Iraq, to military leadership and to Iraqis, and "I believe, in most parts of the world, especially in the Middle East, personal relationships make a difference."

"I believe that this arrangement will probably preserve the likelihood of continued momentum and progress," Gates said.

The defense secretary said he expects to easily win Senate approval for the nominations once they are made by President Bush. He said he had spoken with Sens. Carl Levin, D-Michigan; John McCain, R-Arizona; and John Warner, R-Virginia, and "I don't really anticipate any problems."

In a press release Wednesday, McCain praised the move.

"Both of these great generals have served our country with skill and distinction, and I am pleased that they will continue to do so in positions of high responsibility," McCain wrote.

Gates acknowledged that Fallon's "decision to step down was unexpected."

"I had intended that Adm. Fallon probably stay on for a third year. His second year would have ended in February," he said. "So I'm faced with a critical combatant command where a commander is needed -- and a commander who knows what's going on in the region."

Asked whether he was using Petraeus "almost as a finger in the dike" because of the shortage of Army generals with expertise in counterinsurgency warfare, Gates admitted that there are few Army officers with experience in and knowledge of Iraq.

Odierno served as commander of the 4th Infantry Division, the unit that captured former Iraqi President Saddam Hussein.

Petraeus was picked in January 2007 to replace Gen. George Casey as the chief commander in Iraq and won Senate confirmation that month. He previously served as head of the U.S. Army Combined Arms Center and as a commander in Iraq, and he was one of the main writers of an Army manual on counterinsurgency efforts.

He was seen as the logical choice to replace Fallon. In his new post, he will oversee U.S. forces in all the Middle East, not only Iraq and Afghanistan.

"I recommended him to the president because I am absolutely confident he is the best man for the job," Gates said.

In testimony to Congress this month, Petraeus said Iranian agents have played a "destructive role" in Iraq by backing Shiite militants, called "special groups" by the United States. The U.S. believes that Iran's Islamic Revolutionary Guard Corps Quds Force is behind the support for militants.

"Unchecked, the special groups pose the greatest long-term threat to the viability of a democratic Iraq," Petraeus testified.

Last month, in an interview with CNN's Kyra Phillips in Baghdad, Petraeus acknowledged some "friction" between himself and Fallon in the past year, but "actually, over the last six months or so, our relationship was really very, very good."

"There was friction in the beginning. He has a different job than I have," Petraeus said. "There can be understandable differences of your take, if you will, on a situation. As they say in politics and government 101, where you stand on an issue sometimes depends on where you sit in the organization, and we sit in different chairs." E-mail to a friend

CNN's Jamie McIntyre contributed to this report.

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