Nato secretary general Anders Fogh Rasmussen said on Friday he believed it was "realistic" for foreign forces in Afghanistan to hand over control of security to local forces by 2014.
But in an interview with the BBC, he also insisted that allies were "committed as long as it takes".
Members of the Nato military alliance are meeting in Lisbon on Friday and Saturday for a summit in which they are expected to back the 2014 target for ceding control of the war against Afghan insurgents to Kabul.
"We will make a very important announcement at the summit in Lisbon that a gradual transition to lead Afghan responsibility is about to start at the beginning of 2011 and we hope to see this gradual process completed by the end of 2014, and I find that road map realistic," Rasmussen told BBC radio.
Asked about US plans to start bringing troops home in 2011, he said: "I'm not aware of concrete plans for withdrawal of troops. "On the contrary I think all allies are prepared to stay committed as long as it takes to finish our job."
The Nato chief added: "Our mission will end when the Afghans are capable to take responsibility themselves. "We hope that they will be able to take such lead responsibility all over Afghanistan by the end of 2014.
"But obviously this process must be condition-based. We have to make sure that the Afghans are actually capable to take responsibility before we hand over responsibility to the Afghans.
"But I find it realistic to complete this process by the end of 2014 based on the fact that we see a rapid development of the Afghan security forces."
Rasmussen said there was "concrete progress on the ground right now in Afghanistan — we have sent in more international troops and it has a clear impact on the situation in Afghanistan".