"We think that goal is realistic, and we have made plans to achieve it, but of course if circumstances agree, it could be sooner, absolutely," said Mark Sedwill, NATO's top civilian representative in Afghanistan.

Sedwill said the troop withdrawal starting next year will be "shallow" and eventually accelerate but did not elaborate.

The escalating war in Afghanistan, where the alliance is struggling to contain Taliban militants, looked set to dominate a two-day NATO summit opening Friday in Lisbon.

NATO spokesman James Appathurai also said the alliance's 28 leaders, including President Barack Obama, were poised to approve the withdrawal plan. He said NATO is "quite confident of the end of 2014 timeline for handing responsibility to Afghan security forces" as requested by Afghan President Hamid Karzai.

Appathurai and Sedwill did not say how many NATO troops would stay in Afghanistan after 2014 serving as military advisers and trainers or in a support role.