WASHINGTON — More troops and a new strategy for using them are emerging as critical components to the eight-year-old effort by U.S. and NATO forces to defeat the Taliban and secure Afghanistan.

Defense Secretary Robert Gates, acknowledging bright spots but also "gloom and doom" in a new assessment of the war, said Monday the Obama administration would look closely at requests for resources.

Asking for more troops is not part of the classified assessment delivered by Gen. Stanley McChrystal, the top commander in Afghanistan, according to officials. However, he is expected to ask for more troops in a separate request in a few weeks, two NATO officials told The Associated Press.

McChrystal's assessment is expected to offer a blunt appraisal of the Taliban's increasing tactical prowess and diminishing popular support in Afghanistan for both the foreign-led war effort and the fragile, corruption-riddled central government.

"The situation in Afghanistan is serious," McChrystal said Monday, and success "demands a revised implementation strategy, commitment and resolve, and increased unity of effort."

Gates told reporters traveling with him to Fort Worth, Texas, "We have been very explicit that Gen. McChrystal should be forthright in telling us what he needs."

A Pentagon spokesman said Gates had not yet seen McChrystal's recommendations, which were being reviewed and commented upon by Gen. David Petraeus and others at U.S. Central Command.

Although President Barack Obama committed 21,000 new American forces to Afghanistan this year, officials are bracing for a request for even more. Obama would then face a buildup of troops there just as troop commitments in Iraq are easing or the risk losing the war he argued the U.S. had neglected.

Neither the White House nor Congress would enjoy the prospect of widening the war after eight years and millions of dollars in development money. Violence is escalating: August became the deadliest month of the war with at least 47 U.S. troop deaths.

U.S. and NATO commanders have said they do not have sufficient troops and support to expand the fight against a resilient and well-organized Taliban insurgency.

But Gates noted his oft-repeated worry about placing too many forces in Afghanistan, a strategy that failed for the Soviet Union in the 1980s.

In Brussels, NATO spokesman James Appathurai said the report also would be examined by NATO's political and military leadership.

He stressed it was an assessment by the commander of NATO forces in Afghanistan, "not a change of strategy."

McChrystal recommends focusing the U.S. and NATO counterinsurgency efforts on the Afghan population and less on militants. McChrystal was sent to Afghanistan this summer to oversee the addition of 17,000 U.S. combat forces.