Chicago: NATO will hand over the lead role in combat operations to Afghan forces across the country by mid-2013, alliance leaders said on Sunday as they charted a path out of a war that has lost public support and strained budgets in Western nations.

A NATO summit in Chicago on Monday will formally endorse a US-backed strategy for a gradual exit from Afghanistan, a move aimed at holding together an allied force scrambling to cope with France's decision to withdraw its troops early.

President Barack Obama and NATO partners want to show their war-weary voters the end is in sight in a conflict that has dragged on for more than a decade while at the same time trying to reassure Afghans that they will not be abandoned.

"There will be no rush for the exits," NATO Secretary-General Anders Fogh Rasmussen said as the summit got underway.

He sought to put up a show of unity even as France's new President Francois Hollande vowed to stick by his pledge to withdraw French troops by year's end, two years earlier than the alliance timetable.

NATO's plan is to shift full responsibility to Afghan forces for security across the country by the middle of next year and then withdraw most of the alliance's 130,000 combat troops by the end of 2014, Rasmussen said.

While foreign forces will continue to fight the Taliban and other militants as necessary - and it may be very necessary - the new mission for US and NATO troops will assume a new focus on advising and supporting Afghan soldiers.

Looking toward the November presidential election, Obama - who once called the Afghan conflict a "war of necessity" but is now looking for an orderly way out - sought to dispel the notion that shaky allies will leave US troops to carry the ball alone.

Obama warned of "hard days" ahead as he hosted the summit in his home town, Chicago, a day after major industrialised nations tackled a European debt crisis that menaces the global economy.

The shadow cast by fiscal pressures in Europe and elsewhere followed leaders from Obama's presidential retreat in Maryland to the talks on Afghanistan, an unwelcome weight on countries mindful of growing public opposition to a costly war that has failed to defeat the Taliban in nearly 11 years of fighting.

Obama made clear he expected NATO powers to formally embrace the Afghanistan transition plan, which had already been widely telegraphed by the Pentagon earlier this year.

But the Chicago talks faced undercurrents of division.

Hollande insisted he had no intention of backtracking on a campaign promise for an accelerated troop pullout, which helped him win the presidency from Nicolas Sarkozy this month. He said had reached a "common agreement" on the matter with fellow leaders and he would release details in coming weeks.

A poll in January showed 84 per cent of the French public backed an early troop withdrawal. France has about 3,400 troops in Afghanistan.

"French combat troops will be withdrawn from Afghanistan by the end of the year," Hollande told reporters. "In 2013, only trainers for police and officers of the Afghan army will remain and this will be done within the framework of ISAF."

Hollande's comments underscored the challenge for Obama, who has steadily narrowed his goals in Afghanistan, in plotting a more gradual withdrawal that will not open the way for a Taliban resurgence.

"We went into Afghanistan together, we want to leave Afghanistan together," German Chancellor Angela Merkel told reporters.

Karzai thanks US taxpayers

Obama, meeting Afghan President Hamid Karzai on the margins of the summit, said the conference would agree on a "vision post-2014 in which we have ended our combat role, the Afghan war as we understand it is over, but our commitment to friendship and partnership with Afghanistan continues".

Standing next to Obama, Karzai thanked Americans for "your taxpayer money" and said his country looked forward to the day it is "no longer a burden" on the international community. Karzai's government has been widely criticized for rampant corruption.

Karzai's comments alluded to the political bind that Obama and other Western leaders face in underwriting a unpopular war effort and the build-up of Afghan forces during a time of budget austerity at home.

With heavy security in place for the Chicago summit, baton-swinging police clashed with anti-war protesters marching by the thousands near the summit venue. Lawyers representing the demonstrators said at least a dozen people were injured, some with head wounds from batons, and more than were arrested.

Trying to inject itself into the NATO proceedings, the Taliban urged countries fighting in Afghanistan to follow France's lead and pull their forces out in accordance with anti-war sentiment in the West.

Obama told the summit's opening session: "Just as we have sacrificed together for our common security, we will stand together united in our determination to complete this mission."

Essentially conceding Hollande was unlikely to be dissuaded, General John Allen, the U.S. commander in Afghanistan, played down the impact, saying "we have the capacity, using our current force structure, to ensure there is no degradation in security."

Careful French comments on the issue illustrated the balance NATO leaders must strike as they seek to avoid the appearance of splits with NATO partners without alienating voters who want to see a swift exit.

Alliance leaders walked a cautious line in discussions this weekend on long-term funding for the Afghan police and army, whose ability to battle the Taliban is at the core of NATO strategy for leaving Afghanistan smoothly.

The Obama administration, unwilling to be solely on the hook for the $4.1 billion annual price tag, has been seeking promises from its allies to give $1.3 billion a year for Afghan forces.

While there are few doubts allies will eventually provide support, NATO appeared unlikely to meet that goal by the end of the meeting.

After the first day of the summit, NATO announced a milestone in the effort to provide a pan-European missile defense system, saying it had reached "interim capability." Russia is adamantly opposed to the missile shield, seeing it as a security threat despite US insistence it is meant to defend against Iranian missiles.