"Much has changed over the last two years, causing us to reassess whether we are properly sized to support current operational needs," Gates said, adding the Army would swell to 569,000 soldiers, up from the current 547,000.

"The persistent pace of operations in Iraq and Afghanistan over the last several years has steadily increased the number of troops not available for deployment in the Army."

The military, with President Barack Obama′s "strong support," decided that continuing operations in Iraq and Afghanistan as well as political turmoil in Pakistan, meant its ability to fill troop vacancies was "at risk," Gates said.

According to current deployment estimates, he added, the challenge was a "temporary" one that would peak in the coming year and abate over the next three years.

Obama has ramped up the US effort in Afghanistan, dispatching 21,000 more troops to fight a mounting Taliban-led insurgency just as the United States draws down in Iraq as part of a bilateral agreement signed last year.

Acknowledging the decision meant "additional tough choices" for the Pentagon, Gates said he was "convinced this is an important and necessary step to ensure that we continue to properly support the needs of our commanders in the field while providing relief for our current force and their families."

The Pentagon chief said the additional forces did not necessarily mean the United States would be sending more troops to Afghanistan on top of the fresh reinforcements Obama has ordered.

The new US commander in Afghanistan, General Stanley McChrystal, is set to deliver his assessment to the Pentagon next month and has already indicated he will not shy away from proposing more US troops, regardless of political and economic considerations.

Admiral Mike Mullen, the top US military officer, said he had grown "increasingly concerned over the last year and a half about stress on the force and our ability to meet the demands out there. This temporary increase helps us address that concern."

But he stressed that "it′s not just about relief. It′s about renewing our efforts to fight these two wars."

The Pentagon′s announcement came as senior officers have cited repeated combat tours as a likely factor in the steady rise of suicides in the US military over the past two years.

For years, the suicide rate in the military was lower than among the wider civilian population. But that pattern changed in 2008. Last year, 128 soldiers took their lives, up from 115 in 2007.

The number of suspected suicides in the first half of 2009 reached 88, compared to 67 for the same period last year, according to recently released figures.