US President Barack Obama vowed Friday to stand by Afghanistan after Nato —led troops hand control of the fight against Taliban insurgents to Afghan forces in 2014.

Far from the dusty battlefields of Afghanistan, where more than 2,200 Allied troops have fallen in a vicious guerrilla conflict, Nato leaders gathered in Lisbon for a two-day summit to set a 2014 cut-off date.

By that point, the Western allies want Afghanistan's fledgling forces to take charge of most fighting against insurgent fighters, allowing the bulk of the 150,000-strong US-led force to return home.

"We finally have the strategy and resources to break the Taliban's momentum, deprive insurgents of their strongholds, train more Afghan security forces, and assist the Afghan people," Mr Obama wrote in the Portuguese daily Publico, according to an english-language copy of the article provided to .

Nato leaders will endorse a plan to begin handing security responsibility to Afghan security forces early next year and fulfill Afghan President Hamid Karzai's ambition of giving them full control by 2014.

"And even as America's transition and troop reductions will begin this July, Nato, like the United States, can forge a lasting partnership with Afghanistan to make it clear that, as Afghans stand up and take the lead, they will not stand alone," Mr Obama.

The alliance's summit has been billed as one of the most important in its history, with meetings planned with the beleaguered Afghan leader and former foe Russia's President Dmitry Medvedev.

Karzai is in open disagreement with his Nato sponsors over the tactics to be used in the battle, and Mr Medvedev's Russia has in the recent past been fiercely critical of US missile plans, but leaders hope both can be won over.

Alongside that, they hope to unveil the 28-nation alliance's new "strategic concept", a planning framework to govern how it orders security priorities in a world of multiple new threats and ever tighter defence budgets.

They will unveil plans for a network of radars and interceptors to form an anti-ballistic missile shield in the skies of Europe to protect Nato members, and overcoming Russian concerns by inviting them to take part.

Talks are also planned on means to reform the alliance to slash the number of command headquarters and make them more easily deployable to faraway conflicts such as Afghanistan.

Navigating a way out of the Afghan war, the biggest and longest military operation undertaken by Nato since its creation in 1949 to contain the Soviet Union, is the biggest challenge in Lisbon.

Nato officials insist that the transition is not a rush to the exit door, but the war is deeply unpopular in Europe and governments are under pressure from voters to bring soldiers home.

Mr Obama also faces resistance from his own high command and from newly resurgent republicans in congress.

The pentagon said Thursday the plan to hand over security to Afghan forces in 2014 represents an "aspirational goal", not a rigid deadline.

Press secretary Mr Geoff Morrell said there "may very well be the need" for US-led forces to remain in Afghanistan beyond 2014, albeit in smaller numbers.

The date was chosen by Mr Karzai, who shocked Nato allies this week by urging the United States to scale down military operations and by sharply criticising night raids on Afghan homes.

US defence secretary Mr Robert Gates sought to play down any rift, saying on Tuesday that Mr Karzai's comments reflected "the impatience of a country that's been at war for 30 years".

Mr Karzai, who arrived in Lisbon late Thursday, was to meet with Mr Obama and other western leaders on the sidelines of the summit, which was being held under tight security.

Hundreds of police closed off and secured streets surrounding the complex. A frigate patrolled the river. security forces looked down with binoculars from the city's vasco da gama tower.