"From now until the end of 2014 we will see announcements of redeployments, withdrawals or drawdown," NATO Secretary-General Anders Fogh Rasmussen told British newspaper The Guardian.
"If the security situation allows, I would not exclude the possibility that in certain areas you could accelerate the [withdrawal] process," he said.
Some 68,000 of the 120,000 NATO troops still in Afghanistan are American.
Rasmussen said the decision on withdrawing troops, and speeding up the transfer of primary security responsibilities to Afghan troops, would be made after U.S. Marine Corps four-star Gen. John R. Allen, the top commander in Afghanistan, delivers his military recommendations in the next two months.
"Political decisions will be taken, based on his recommendations, as to how we will adapt to the transfer of lead responsibility to the Afghans," he said. "The pace will very much depend on the security situation on the ground."
The U.S. military last month fully withdrew the last of the 33,000 "surge troops" sent to pacify Afghanistan two years ago.
Rasmussen acknowledged the killings of at least 52 allied troops this year by Afghan security forces in insider, or green-on-blue, attacks -- green being U.S. military jargon for indigenous forces, blue for its own -- had damaged the relationship between the international forces and Afghanistan's police and military.
"There's no doubt insider attacks have undermined trust and confidence, absolutely," he told the newspaper.
Rasmussen spoke with The Guardian at the intergovernmental military alliance's headquarters in Brussels after announcing NATO forces resumed almost all operations with Afghan forces after cutting them back Sept. 18 because of the rise in insider attacks.
Lt. Gen. Adrian Bradshaw, deputy commander under Allen, said at the time the cutbacks also reflected increased caution following violent reactions in the Muslim country to an incendiary video posted on YouTube that denigrated the Prophet Muhammad.
Insider attacks have increased greatly this year compared with 2011, when there were 35 over 12 months.
France announced it was leaving by the end of next year -- a year earlier than originally planned -- after four of its soldiers were killed in a January insider attack.
In that attack, a rogue Afghan soldier fired on unarmed French troops embedded with Afghan forces on a training mission in the northeastern Kapisa province. Another 15 French soldiers were wounded, eight of them seriously, France said.
Rasmussen told the newspaper the insider attacks were not confined to rogue soldiers or disgruntled and embittered Afghan security forces with grudges against their Western mentors.
He said they had more to do with a Taliban strategy of infiltrating Afghan security structures and then sowing distrust and confusion.
"It's safe to say that a significant part of the insider attacks are due to Taliban tactics. ... Probably it is part of a Taliban strategy," he said of the Islamic fundamentalist militant movement.
The U.S. death toll from the 11-year Afghan campaign is at least 2,000.