President Barack Obama unveiled a strategy on Thursday for a leaner US military focused on countering China's rising power and signaling a shift away from large ground wars against insurgents.

The plan calls for preparing for possible challenges from Iran and China, requiring air and naval power, while virtually ruling out any future counter-insurgency campaigns such as those conducted in Iraq and Afghanistan.

The 'defense strategic review' sets out an approach for the US military in a looming era of austerity, as Obama's administration prepares for $487 billion in defense cuts over the next 10 years.

But the US president, anticipating attacks from his Republican rivals in an election year, said reductions would be limited and would not come at the expense of America's military might.

"So yes, our military will be leaner, but the world must know -- the United States is going to maintain our military superiority with armed forces that are agile, flexible and ready for the full range of contingencies and threats," Obama told reporters in a rare appearance at the Pentagon.

White House officials said Obama was deeply involved in the strategy review and sought to portray the president as taking a careful approach to defense spending, based on the advice of leading commanders.

Saying the country was "turning the page on a decade of war," Obama said the new strategy would increasingly focus on Asia, where commanders worry about China's growing military power.

"We'll be strengthening our presence in the Asia Pacific, and budget reductions will not come at the expense of this critical region," he said.

Defense Secretary Leon Panetta, appearing with Obama along with top officers, said the strategy envisages a 'smaller and leaner' force that will expand the military's role in Asia while maintaining a strong presence in the Middle East.

According to the eight-page strategy document, the military will work with allies in the Middle East to ensure security in the Gulf and prevent Iran from securing nuclear weapons.

However, counter-insurgency operations receive a lower priority under the plan, enabling the administration to scale back ground forces.

Panetta said "with the end of US military commitments in Iraq, and the drawdown already under way in Afghanistan, the Army and Marine Corps will no longer need to be sized to support the large scale, long-term stability operations that dominated military priorities and force generation over the past decade."

The US military's top officer, General Martin Dempsey, praised the strategy but acknowledged it carried some risks, which could in some cases mean a slower response or fewer resources for an operation.

"We do accept some risk, as all strategies must. Because we will be somewhat smaller, these risks will be measured in time and capacity," the general said.

But he said the country faced "tough economic times" and had to adapt to new threats. The chairman of the House Armed Services Committee, Buck McKeon, hit out at the strategy and accused Obama of gutting defense.

"The President has packaged our retreat from the world in the guise of a new strategy to mask his divestment of our military and national defense," McKeon, a Republican, said in a statement.

The review reinforces what defense officials have already signaled -- that funds will flow to aircraft, ships, missile defense and high-tech weaponry while the US Army and Marine Corps will be downsized.

Washington's focus on Asia is fueled by concerns over China's growing navy and arsenal of anti-ship missiles that could jeopardize America's military dominance in the Pacific.

In keeping with plans for a smaller force, the strategy discards the doctrine that the military must be prepared to fight two wars at the same time, an idea long debated inside the Pentagon.

Instead, the United States would be ready to fight one war while waging a holding action elsewhere to stave off a second threat.

The strategy review suggests reducing the atomic arsenal without saying how, amid calls from some lawmakers to reduce the number of nuclear-armed submarines.

The review also hints at scaling back the military's footprint in Europe but offered no details, saying 'our posture in Europe must also evolve'.

Britain's defense minister cautioned Thursday the US pivot to Asia should not neglect Russia, which he called an unpredictable force on the global stage.

The new strategy comes ahead of the proposed defense budget for 2013 due to be released next month, which is expected to call for delays in some weapons programs, including the troubled F-35 fighter.

Despite talk of belt-tightening, the defense budget for 2012 came to $530 billion, not counting the cost of the war in Afghanistan.

Obama said future military spending will still remain high and 'larger than roughly the next 10 countries combined'.