The US military's operations in Iraq and Afghanistan have constrained its ability to tackle other potential conflicts, making any future war more likely to be longer and bloodier, according to America's top general.

In an annual classified report required by Congress, Air Force Gen. Richard Myers, chairman of the Joint Chiefs of Staff, said stress on manpower and equipment could limit the ability to win other possible wars as quickly as the Pentagon had previously forecast, defence officials said on Tuesday.

Myers stated in the report that US armed forces would "succeed" in any future major conflict but "may be unable to meet expectations for speed or precision."

Any future armed conflicts "may result in significantly extended campaign timelines, and achieving campaign objectives may result in higher casualties and collateral damage," the report stated.

Potential hot spots include Iran, the Korean Peninsula and across the Taiwan Strait.

Myers, due to step down from his post in September, said the report showed "we have very high standards in how we measure ourselves."

"The timelines may have to be extended. And we may have to use additional resources. But that

doesn't matter because we're going to be successful in the end," Myers told reporters.

The United States has about 138,000 troops in Iraq and another 17,000 in Afghanistan, and has shifted troops from such places as South Korea, where they guarded against aggression by communist North Korea, to maintain force levels to combat the Iraqi insurgency.

There are 1.4 million active-duty US troops and another 1.2 million in the Reserve and National Guard.

The White House emphasised that the report maintained the US military remained fully capable of meeting any threat posed in the world.