“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 set 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.

U.S. Secretary of State Hillary Rodham Clinton, who arrived in Lisbon ahead of Obama, met for an hour Friday with Karzai, a State Department official said.

The official, who spoke on condition of anonymity to discuss a private meeting, said the session at Karzai’s hotel was “candid and friendly” and covered key aspects of the NATO mission in Afghanistan — including the planned transition to Afghan security control, as well as international civilian assistance to Kabul.

They reached a “common understanding” on outlines of a longer-term Afghan-NATO partnership, the official said.

NATO officials say they expect unanimous support from the allies for Obama’s plans for a new, expanded missile defense system in Europe that would be based on an existing shield meant to defend military units from attack. The U.S. already has a missile defense system based mainly in North America, and it is planning one for its European allies.

But Obama will face tough questions from U.S. allies on his exit strategy in Afghanistan. He will also meet with leaders of the European Union on Saturday to defend his preference for stimulus spending at a time when many European nations are enacting economic austerity measures.

The NATO leaders are expected on Saturday to endorse the plan by Gen. David Petraeus, the top U.S. and NATO commander in Afghanistan, to start handing over responsibility for security in some areas of Afghanistan to government forces next year.

Obama told El Pais, Spain’s leading newspaper, he expected the allies will pledge additional trainers for Afghan security forces.

“This effort is going to take time and our commitment to Afghanistan and the Afghan people is for the long-term,” Obama said. “We cannot turn our backs on the Afghan people.”

Clinton defended the high cost that European nations are paying for their participation in the war in Afghanistan, and urged them to stay the course despite dire economic difficulties for many countries that have translated into wage cuts, lost jobs and massive government budget reductions.

“Though we are very supportive of the difficult decisions that will have to be made concerning the economy, just as back home President Obama is making difficult decisions concerning our own economy, we believe that the mission we are pursuing in Afghanistan must continue,” Clinton told reporters.

The alliance has 140,000 troops in Afghanistan, two-thirds of them Americans. The government’s security forces are being built up to just over 300,000 members. Their Taliban opponents are estimated to number up to 30,000 men.

Allied commanders have highlighted their successes this year against Taliban insurgents in Helmand and Kandahar provinces, to emphasize that transition is ready.

But allied casualties have also reached record levels of some 650 dead this year, and the Taliban have spread out into other parts of Afghanistan.

On other issues, NATO’s newly expanded anti-missile shield would cost €200 million ($273 million) over the next 10 years, according to NATO chief Fogh Rasmussen, who also wants Russia to cooperate in the project. Despite claims by protesters that debt-plagued Europe can’t afford it amid austerity cuts, alliance officials insisted the project is worth it.

“We think it’s a good thing to have a missile defense system which is NATO-based,” Britain’s Defense Secretary Liam Fox told BBC Radio 4’s Today program. “That provides us with communal protection over the years ahead, it’s cost-effective for us, and there are some 30 countries now which either have or are developing ballistic missiles.”

NATO’s leaders will not explicitly identify any potential enemy, although in the past officials have publicly singled out Iran and its ballistic missile program. But alliance member Turkey, which maintains close ties with Tehran, refused to let NATO name Iran as a threat.

“We cannot accept that any specific country (including) our neighbor Iran to be shown as a target,” Turkish President Abdullah Gul said. “It is absolutely out of the question.”

Founded in 1949 to counter the threat of a Soviet invasion, the 28-member alliance is in the midst of a mid-life crisis as it searches for relevance almost 20 years after the collapse of its communist rival.

Other elements of NATO’s new mission statement expected to be adopted Friday include new roles such as cyber-warfare and missions outside NATO’s traditional area in Europe, such as anti-piracy patrols off the Somali coastline.

NATO’s previous strategic concept focused mainly on its peacekeeping role in places like Bosnia and Kosovo. It was adopted in 1999, soon after the end of the Cold War and before the Sept. 11, 2001, attacks in the United States forced the alliance to take on missions such as counterinsurgency warfare in Afghanistan.

The new document will also warn European governments not to slash defense spending at a time of economic crisis, because of the growing discrepancy in military capabilities between the United States and Europe’s NATO members. Most European nations are not even meeting the minimal requirement of devoting 2 percent of their GDP to defense.

America’s latest defense budget of over $710 billion dwarfs the combined annual military expenditures of its European allies, which total about $280 billion.

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Barry Hatton and Robert Burns contributed from Lisbon.