He was speaking following the appointment of US General David Petraeus as top Afghan war commander in place of General Stanley McChrystal, fired by President Barack Obama for disparaging remarks about administration officials.
"I can assure the Afghan people that we'll stay as long as it takes to secure their country," he added, noting that Petraeus had given a similar message.
"Obviously that's not forever," he said. "Our mission we'll end when the Afghans are capable to secure their own country themselves."
Rasmussen was on his first visit to Portugal as NATO chief to discuss with local officials, including Prime Minister Jose Socrates, the summit of the alliance to be held in Lisbon at the end of November.
"I hope that we will be able to make an announcement at the summit that transition can start in some provinces (of Afghanistan) but of course conditions must be fulfilled." he said.
"This is a reason why our training mission in Afghanistan is so important. We have to train and educate Afghan soldiers and Afghan police."
Speaking after his talks with Socrates, Rasmussen described the summit of the alliance's 28 member states on November 19 and 20 as one of the most important in NATO's history.
"We're going to approve a new strategic concept, a document which will describe the core tasks of NATO in the next 10 years," he said.
"The core functions will remain collective defence of our populations and our territories, but we also have to make sure that we can protect our populations against the new threats like cyber-attacks and missile attacks."
Source: AFP Global Edition