Nato to debate Afghanistan at crucial Lisbon summit
The 28 member states are hoping to reach a "New Strategic Concept" to shape the way Nato defends itself against threats over the next decade.
Russian President Dmitry Medvedev will also attend, in a sign of warming ties.
Afghanistan will be top of the agenda, with plans to bring Nato's combat operations to an end by 2014.
Afghan President Hamid Karzai, who is scheduled to address the summit on Saturday, has said he wants Nato to hand back control of the country by the end of 2014 - a deadline the US has described as realistic but not set in stone.
Pentagon press secretary Geoff Morrell said the deadline had existed for some time as "an aspirational goal" but that this did not mean all coalition forces would have to leave by that date.
The Lisbon talks are expected to shape the future of Nato at a time of shrinking budget cuts and expanding challenges, says the BBC's defence correspondent, Caroline Wyatt.
See www.bbc.co.uk for full version.