The US president has warned of "hard days ahead" at a NATO summit in Chicago dominated by the forthcoming withdrawal of international forces from Afghanistan.
"We still have a lot of work to do and there will be great challenges ahead," Barack Obama said on Sunday. "The loss of life continues in Afghanistan and there will be hard days ahead."
The aim of the NATO summit is to agree on a common stance as the alliance prepares to hand over security duties to Afghan forces at the end of 2014.
More than 50 leaders are attending the two-day meeting, including heads of state and government from the 28 NATO countries, as well as Afghan President Hamid Karzai and Asif Ali Zardari, his Pakistani counterpart.
"Afghanistan will be no longer a burden on the shoulders of our friends in the international community, on the shoulders of the United States and our other allies."
- Hamid Karzai, Afghan president
Meanwhile, scuffles between police and protesters broke out after thousands of anti-war demonstrators marched as close as they were allowed to the summit venue.
The protest was largely peaceful until the end when a small group of demonstrators clashed with a line of police who tried to keep them from the lakeside convention centre where the meeting was taking place.
The marchers were led by a group of US veterans of the wars in Iraq and Afghanistan who symbolically gave back their military medals.
Al Jazeera's John Hendren, monitoring the protests in Chicago, said several people had been arrested and handcuffed.
"Some protesters walked away with blood on their faces so it appears it turned violent in a minor way," he said.
'Peace and security'
Obama said that the world was behind his strategy to end the war in Afghanistan. As he met Karzai, he also said that the United States recognised the "hardship" Afghanistan had been through, adding that its people "desperately want peace and security."
Karzai said it was important to complete the security transition and withdrawal of foreign combat troops from Afghanistan that the summit is expected to ratify.
"Afghanistan will be no longer a burden on the shoulders of our friends in the international community, on the shoulders of the United States and our other allies," he said.
New French President Francois Hollande has promised to pull his country's soldiers out of Afghanistan by the end of this year.
While insisting he remained committed to NATO, Hollande told reporters he would "ensure our soldiers come back before the end of 2012.
But NATO Secretary-General Anders Fogh Rasmussen expressed confidence the alliance would "maintain solidarity" despite France's decision. "There will be no rush for the exits," he said.
He also said he was optimistic that the international community would continue to finance the Afghan security forces, adding that supporting Afghan forces was less expensive than deploying NATO troops.
Rasmussen said that the international community in general had a responsibility and interest in ensuring that Afghan forces took full responsibility for security after 2014 to avoid terrorist safe havens being re-established.
Some nations, including the US, Australia, Britain, and Germany, have made pledges to an international fund set up to help Afghan forces after the NATO pullout.
Al Jazeera's Patty Culhane, reporting from Chicago said: "US officials are not saying what they will be pledging, as it seems they want other countries to pledge initially."
The US is expected to pay half of an estimated $4bn needed every year.
A sticking point for NATO's campaign in Afghanistan remains Pakistan's refusal to reopen supply routes into Afghanistan since closing them in protest over a US airstrike which killed 24 soldiers last November.
Zardari's presence in Chicago had raised hopes of an imminent deal, but so far no announcement has so far been forthcoming.