US President Barack Obama has warned of "hard days ahead" at a Nato summit in Chicago dominated by the issue of withdrawal from Afghanistan.
France's new President Francois Hollande has again said he will pull French troops out by the end of 2012, nearly two years ahead of schedule.
Nato is preparing to hand over security to Afghan forces by the end of 2014.
Afghan President Hamid Karzai said his country was fully aware of its responsibilities.
Some Nato members have pledged aid to help Afghan forces tackle the Taliban insurgency on their own.
The BBC's defence correspondent Caroline Wyatt says the summit is seeking to reconcile two different messages.
It is telling the public in Nato countries that the fighting in Afghanistan is coming to an end for their troops, while reassuring the Afghans that the alliance will not abandon them after 2014, our correspondent says.
The meeting continues for a second day on Monday with discussion about how much money is needed to continue to support Afghanistan and who pays.
Talks are also due to take place with President Asif Ali Zardari of Pakistan over the re-opening of supply routes through his country to Afghanistan.
"There will be no rush for the exits," Nato Secretary General Anders Fogh Rasmussen said on Sunday. "Our goal, our strategy, our timetable remain unchanged."
Mr Obama warned there were still "great challenges ahead", urging leaders to "pool resources".
"Just as we've sacrificed together for our common security, we will stand united in our determination to complete this mission," Mr Obama told the summit, which is taking place in his home city.
More than 50 leaders are attending the summit, including heads of state and government from the 28 Nato countries, along with President Karzai and and Pakistani leader President Zardari.
As talks began, President Obama spoke of a "transformational decade" in Afghanistan and the enormous sacrifices of the American people on the road to peace, stability and development.
In addition to Mr Hollande, several Nato leaders are under domestic political pressure to withdraw troops from Afghanistan before 2014.
However, Mr Rasmussen said such moves were part of the plan, rather than a contradiction.
"We are now in a process of gradually handing over lead responsibility for security to the Afghans and that process will be completed by the end of 2014 and during that process you will see withdrawal of troops, a shift from combat to support," he told the BBC.
US officials said the alliance was expected to formally endorse a plan to put Afghan troops in charge of combat operations by mid- to late 2013.
Some nations - including the US, Australia, Britain, and Germany - have pledged to contribute to an international fund to help Afghan forces after the Nato pullout.
The US is expected to pay half of an estimated $4bn (£2.5bn) needed every year.
President Karzai, standing alongside President Obama, thanked Americans for their financial help and said his country looked forward to the day that it was "no longer a burden" on the international community.
More than 10 years after the US toppled the Taliban regime, violence is continuing unabated in Afghanistan. According to UN figures, the number of deaths reached a record 3,031 in 2011 - the great majority caused by militants.
Earlier this month, the Taliban announced the start of their annual spring offensive. On Saturday, a suicide bomber killed at least 10 people, a number of them children, at a checkpoint in the eastern province of Khost.
Missile defence row
Correspondents say the Obama administration is hoping that President Zardari will agree to reopen key Nato supply routes through Pakistan into Afghanistan, which were closed in November after US air strikes killed Pakistani troops.
Pakistan's co-operation is regarded as key to the success of the international mission in Afghanistan, as 130,000 US-led troops fight a Taliban insurgency.
Also on the agenda at the Nato summit are plans for a US-led missile defence system for Europe, aimed at countering a possible threat from Iran.
The leaders are expected to announce the first phase of the scheme, with the deployment of US warships armed with interceptors in the Mediterranean and a radar system based in Turkey.
Russia has voiced strong opposition to the plan, saying it undermines the value of its nuclear deterrent.
The summit is taking place amid heavy security.
Outside the conference, riot police clashed with demonstrators protesting about war, climate change and a wide range of other issues.
Witnesses said some had blood streaming down their faces. Police were seen carrying some people away from the scene.
The protest march through Chicago had been largely peaceful until the end when a small group of demonstrators tried to break through police lines and reach the lakeside convention centre where the summit is being held.