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.
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 his Pakistani counterpart Asif Ali Zardari.