LISBON (Reuters) - NATO will back plans to pull most of the 150,000 foreign troops out of Afghanistan within four years at a summit starting on Friday, although Afghan forces may not be ready to defend the country by then.

NATO leaders will announce the exit strategy from a war widely seen as going badly for the United States and its allies, but approve a new 10-year vision statement underlining the need for the alliance to be ready for similar missions in the future.

At a two-day summit in Lisbon, the 28 NATO leaders and their allies will formally endorse a timetable to start handing over security responsibility to Afghan forces next year and for them to take responsibility countrywide by the end of 2014.

The alliance aims to switch to a training and support role over the four years although NATO’s senior civilian representative in Kabul, Mark Sedwill, said this week that poor security in some areas could push back the 2014 target date.

Afghanistan could still face “eye-watering levels of violence by Western standards” after 2014, he said, despite upbeat NATO assessments about anti-insurgent operations since a big U.S.-led troop increase last year.

British Prime Minister David Cameron, whose country has suffered increasing casualties in Afghanistan, said British troops must be out by 2015.

“I think the British public deserve to know there is an end point to all this, it is 2015, and that’s clear,” Cameron told a parliamentary committee. He said British troops would by 2015 have “made a massive contribution, made massive sacrifices.”

A DECADE OF CONFLICT

The U.S.-led intervention began in response to the September 11, 2001, attacks on the United States after the Taliban refused to hand over al Qaeda leader Osama bin Laden. Now in its 10th year, it has become a political problem for President Barack Obama.

A war effort costing billions of dollars a week has failed to stop a Taliban insurgency spreading through the country and foreign casualties have hit record levels.

Western officials no longer talk of defeating the Taliban, and speak instead of a political solution. Attempts to encourage insurgent leaders to make peace have failed to bear fruit, and the Taliban says foreign troops must first withdraw.

Fears of a quagmire like that suffered in the Vietnam War have sparked a fierce policy debate in the United States. In agreeing to military calls for a big troop increase last year, Obama said forces would start coming home next July, prompting European leaders to look at similar timetables.

However, the exit plan is fraught with difficulties and few military analysts predict a satisfactory conclusion.

The strategy hinges on effort to build up Afghan forces so they can contain the widening insurgency, with a target strength set at more than 300,000 by the end of 2011.

But this has been hampered by high desertion rates and the Kabul government is widely seen as too corrupt, unstable and inept to survive long without foreign military support.

Frustration with Kabul was evident this week when media said U.S. and NATO commander General David Petraeus had accused President Hamid Karzai, who will attend the Lisbon summit, of undermining the war effort by saying that raids by foreign forces were inciting Afghans to join the Taliban.

NEW NATO CONCEPT

Despite NATO’s difficulties in Afghanistan, where a perceived failure would undermine its prestige, alliance leaders will recommit to a global military role when they adopt a new Strategic Concept for the coming decade.

NATO Secretary-General Anders Fogh Rasmussen says the North Atlantic Treaty Organization is the only alliance capable of responding to global crises.

Even so, military analysts say the Afghan experience and pressure on military spending since the global financial crisis have reduced enthusiasm in the United States and other countries for “out-of-theater roles” -- operations in non-NATO countries.

Rasmussen has urged states to “cut fat, not muscle,” and to produce a more agile alliance to dealing with modern threats.

The leaders will commit to bolstering resources for overseas operations such as helicopters, air transport and technology against roadside bombs.

They are also expected to agree to extend a missile defense system and underline the importance of cooperation with strategic partners such as Russia, through which NATO wants help to broaden its supply routes to Afghanistan.

President Dmitry Medvedev meets NATO leaders on Saturday. NATO sees this meeting as a chance to improve ties damaged by Russia’s 2008 intervention in Georgia.