U.S. President Barack Obama walks through the Colonnade of the White House in Washington on Thursday. (Manuel Balce Ceneta/Associated Press)

President Barack Obama says he'll give U.S. troops a strategy and a clear mission if he decides to put their lives at risk protecting the American people or U.S. interests.

Obama addressed troops at Elmendorf Air Force Base in Alaska on Thursday. The president spoke as Air Force One refuelled before he heads to Tokyo for his first visit to Asia.

Obama told thousands of troops and their loved ones that he will never hesitate to use force to protect America or its interests. But if doing so is necessary, he promised them a strategy, a clear mission and the equipment and public support they need to do the job.

Earlier Thursday, Defence Secretary Robert Gates said Obama is close to making a decision about whether to send thousands more troops to Afghanistan.

Speaking to reporters accompanying him on a domestic trip, Gates said Obama did not choose any of the specific options laid out for him at a White House meeting on Wednesday, pushing instead for revisions to clarify how and when U.S. troops would turn over responsibility to the Afghan government.

That stance comes in the midst of forceful reservations about a possible troop buildup from the U.S. ambassador in Afghanistan, Karl Eikenberry, according to a second top administration official.

In strongly worded classified cables to Washington, Eikenberry said he had misgivings about sending in new troops while there are still so many questions about the leadership of Afghan President Hamid Karzai.

Obama is expected to announce his revamped war strategy soon — most likely after he returns from a trip to Asia that ends on Nov. 19.

Increase still likely

Military officials said Obama has asked for a rewrite before and resisted what one official called a one-way highway toward war commander Gen. Stanley McChrystal's recommendations for more troops. The sense that he was being rushed and railroaded has stiffened Obama's resolve to seek information and options beyond military planning, officials said, though a substantial troop increase is still likely.

The president was considering options that include adding 30,000 or more U.S. forces to take on the Taliban in key areas of Afghanistan and to buy time for the Afghan government's small and ill-equipped fighting forces to take over. The other three options on the table Wednesday were ranges of troop increases, from a relatively small addition of forces to the roughly 40,000 that the top U.S. general in Afghanistan prefers, according to military and other officials.

The key sticking points appear to be timelines and mounting questions about the credibility of the Afghan government.

Administration officials said Wednesday that Obama wants to make it clear that the U.S. commitment in Afghanistan is not open-ended. The war is now in its ninth year and is claiming U.S. lives at a record pace as military leaders say the Taliban have the upper hand in many parts of the country.

Eikenberry, the top U.S. envoy to Kabul, is a prominent voice among those advising Obama, and his sharp dissent is sure to affect the equation. He retired from the army this year to become one of the few generals in American history to switch directly from soldier to diplomat, and he himself is a recent former commander of U.S. troops in Afghanistan.

Eikenberry's cables raise deep concern about the viability of the Karzai government, according to a senior U.S. official familiar with them who spoke on condition of anonymity to discuss the classified documents. Other administration officials raised the same misgivings in describing Obama's hesitancy to accept any of the options before him in their current form.

The options presented to Obama by his war council will now be amended.

Military officials say one approach is a compromise battle plan that would add 30,000 or more U.S. forces atop a record 68,000 in the country now. They described it as "half and half," meaning half fighting and half training and holding ground so the Afghans can regroup.

The White House says Obama has not made a final choice, though military and other officials have said he appears near to approving a slightly smaller increase than McChrystal wants at the outset.

Among the options for Obama would be ways to phase in additional troops, perhaps eventually equalling McChrystal's full request, based on security or other conditions in Afghanistan and in response to pending decisions on troops levels by some U.S. allies fighting in Afghanistan.