The comment was the latest indication that the U.S.-led military operation in Afghanistan will continue to be sizable well into the next decade, despite plans to draw down troops and transfer responsibility to the Afghan government.
Mark Sedwill said the end of 2014 was not a deadline. "It's a goal," he told reporters in the capital. "It's realistic but not guaranteed."
He said the transition to Afghan control of security will be slow and piecemeal - often starting with individual districts and building up to the province level. Each area will be evaluated for transfer based on four criteria - the security situation, the capacity of Afghan security forces in the area, the preparations of NATO forces and the progress toward governance reforms.
As a result, some areas could still be very much held and secured by NATO forces years after the benchmark passes.
"There might still be one or two parts of the country where the transition process is ongoing and that might last into 2015 or beyond," said Sedwilll, NATO's senior civilian representative. "This is the point about 2014, it's not an end of mission. It's not even a complete change of mission, but it is an inflection point where the balance of the mission would have shifted."
Two-thirds of all enemy-initiated attacks occur in three provinces - Kandahar and Helmand in the south and Kunar in the northeast, so those areas will likely be the last to be handed over, NATO officials have said, with 10 Afghan districts accounting for 50 percent of all the violence.
In addition, specialist strike units that target terrorist operatives are likely to keep conducting operations even after the Afghan government has taken over responsibility, Sedwill said.
He said both 2011 - the date set for U.S. troops to begin drawing down - and 2014 are "intermediate milestones" in a larger mission that will last much longer.
A summit of Afghan leaders and allies Lisbon this weekend will aim to set broad terms for that longer mission. NATO and the Afghan government plan to sign an agreement that will set out the international community's commitment to Afghanistan, including support for training forces, the growth of the Afghan military and intelligence sharing, Sedwill said.
"We want to build the Afghan leadership so they're taking more and more responsibility for themselves but we recognize it has got to be underwritten by long-term international commitment," Sedwill said.
The Lisbon summit will be the third and largest international meeting on Afghanistan this year as the country's Western allies have come under increasing pressure to provide exit strategies that show timelines for leaving Afghanistan or at least shifting to a mainly training mission.
Both the Afghan government and NATO nations have said they're committed to making this transition happen, but they've been hampered this year by increasing violence, with NATO deaths climbing and insurgents expanding attacks to previously peaceful areas in the north and west.
Even with the NATO mission appearing to stretch out longer and longer, Sedwill said that the momentum had shifted in NATO's favor.
"It's still clearly fragile. There are significant risks and there will be a long and hard campaign ahead, but we believe that in 2010 we have achieved what we wished to, which is that we've regained the initiative - having, candidly, lost it in the past few years," Sedwill said.
He said that this assessment was the conclusion of an assessment of the Afghan campaign that he had conducted with Gen. David Petraeus, the top commander of U.S. and NATO forces in Afghanistan.