|Nato leaders and their allies are expected to endorse a gradual exit plan which may exceed the 2014 deadline [AFP]
The Nato military alliance is expected to back plans for a gradual withdrawal of most of the 150,000 foreign troops from Afghanistan within four years.
The target for handing over security to Afghan forces by 2014 will top the agenda at a two-day summit of the defence bloc in Portugal's capital city, Lisbon, this weekend.
Barack Obama, the US president, departed from Andrews Air Force Base in Maryland for the summit on Thursday - his two days of diplomacy likely to be framed by back-to-back summits: one with the North Atlantic Treaty Organisation and then a joint US-European Union gathering.
The US defence department said on Thursday the plan for gradual withdrawal from Afghanistan represented an "aspirational goal" rather than a rigid deadline.
A Pentagon spokesman said the US was hopeful that Afghan forces would be able to take the lead for security across the country within four years, but that foreign forces might remain beyond the target date.
"So, 2014 has been out there for quite some time as an aspirational goal for us to meet in terms of ultimately putting the Afghan security forces in the lead, having primary responsibility for the security of their country," Geoff Morrell, the Pentagon's press secretary, said.
Morrell's comments echoed remarks on Wednesday from Mark Sedwill, Nato's civilian representative in Afghanistan, who also portrayed the 2014 date as "realistic but not guaranteed".
The alliance aims to switch to a training and support role over the four years, but Sedwill said that poor security in some areas could push back the 2014 target date.
Al Jazeera's James Bays, reporting from Lisbon, said the summit will have a "very wide agenda" and that Nato members will be discussing the future and role for the alliance.
"They are going to come up with a new strategic concept, as they call it," he said.
"That will talk about things like counter-terrorism, like piracy, cyber threats - all of these will be taken into account in this new strategic concept which, in layman's terms, is a new blueprint for what Nato does."
But Beverly Crawford, a professor of political science at the University of California, Berkeley, told Al Jazeera that the Lisbon meeting may not achieve much as Nato faces "huge problems".
|Thousands of anti-Nato activists are planning to stage a protest march during the weekend summit [EPA]
"Economic problems and economic crises throughout Europe and throughout the United States are stretching [Nato's] capabilities very thin," she said.
"And as they get stretched thin, especially in Europe, the gap between Europe and the United States grows bigger and bigger all the time, and at the same time Afghanistan is sapping Nato's credibility right now."
There are 46 countries with troops in Afghanistan, totalling nearly 131,000 soldiers, most of them from the US, Britain, Germany, France, Italy, and Canada.
Nato leaders and their allies attending the summit are expected to formally endorse a timetable to start handing over security responsibility to Afghan forces next year.
"I think that, seen retrospectively, we underestimated the challenge and our operation in Afghanistan didn't have sufficient resources, and yes, that was a mistake," Anders Fogh Rasmussen, the Nato secretary-general, said in comments broadcast by Portugal's Renascenca on Thursday.
"We'll make a positive announcement in Lisbon, that the handover is about to begin."
Anti-Nato activists have been staging peaceful protests in Lisbon against Nato's military presence in Afghanistan, with thousands more expected to arrive for a protest march on Saturday.
The number of Afghan civilians killed in the conflict rose by a third in the first six months of 2010 to 1,271, with most deaths caused by anti-government attacks, the UN said in August.
The leaders of the 28 Nato member nations are also expected to commit to bolstering resources for overseas operations, agree to extend a missile defence system and underline the importance of co-operation with strategic partners such as Russia, through which Nato wants to broaden its supply routes to Afghanistan.
The exit strategy hinges on the ability to build-up Afghan forces, with a target strength of more than 300,000 by the end of 2011.
Nato's training of about 134,000 Afghan police and 170,000 soldiers by next October is seen as vital towards ensuring the withdrawal of foreign troops.
But this has been hampered by high desertion rates and the Afghan government is widely seen as too corrupt, unstable and inept to survive long without foreign military support.
Al Jazeera's Laurence Lee, reporting from Lisbon, said Nato leaders might consider the idea of first handing back the quieter parts of Afghanistan to local law enforcement with the tougher to control areas following later.
"But they will not say this weekend which bits might go first and which might follow ... partly because it's still four years away and partly because they don't want to give the Taliban any clues [as to the exit strategy].
Our correspondent said the bigger, strategic question of Nato's future role beyond Afghanistan will also be raised at the summit.
"As a defensive bloc, Nato needs to have what is perceived to be an enemy on behalf of all its members states," he said. "So the question post-Afghanistan is, who is the enemy?"
Crawford, the University of California, Berkeley, professor, said Nato's identity was in doubt and that it has been for the past 20 years.
"Nato was formed in 1949 as a defensive alliance against the Soviet Union," she told Al Jazeera.
"When the Soviet Union fell, Nato searched for a political identity as a political actor, as a political organisation and then Nato took on the role of out-of-area missions. And it seems to me that it's role has been stretched extremely far now."