Nato to debate Afghanistan at crucial Lisbon summit
Nato members are preparing to meet in Portugal for what is being billed as one of the most crucial summits in the alliance's 61-year history.
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.Alliance credibility
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.
Key to the future credibility of the alliance will be ensuring a workable transition in Afghanistan, our correspondent adds.
On Thursday, Nato Secretary General Anders Fogh Rasmussen said the alliance had "underestimated the challenge" in Afghanistan but was confident it was now "on the right track".
"I'm very optimistic about our Afghanistan operation and we'll make a positive announcement in Lisbon - that the handover is about to begin," he told Portugal's Renascenca newspaper.
There are some 120,000 international troops attached to the Nato-led International Security Assistance Force (Isaf) in Afghanistan.Budget resignation
Mr Medvedev will meet the leaders on Saturday, becoming the first Russian president to attend a Nato summit since his country's conflict with Georgia in 2008.
The alliance is keen to build bridges with Moscow, and a key issue at the summit will be agreeing plans for a joint study of missile defence.
The efforts have been aided by US President Barack Obama's insistence that the US will ratify a new nuclear arms treaty with Russia
He said there was "no higher national security priority" for the government before the start of the new Congress in January.
Moscow is also promising logistical help for Nato in Afghanistan by easing restrictions on transit routes into the country.Spy chief resigns
The summit will also debate proposals on changing Nato's command structure, in an attempt to reduce bureaucracy and expenditure.
The changes could see the number of Nato agencies which look after areas such as logistics, communications, research and training cut from 14 to three.
Meanwhile Portugal's Defence Minister Augusto Santos Silva has said the resignation of the country's spy chief will have no effect on security or intelligence gathering.
Jorge Silva Carvalho resigned on Thursday in protest over budget cuts being imposed by the government.
He is reported to have told colleagues he was quitting "to draw attention to the mistake that is being made" in closing seven of the agency's overseas bureaux.