The retreat of western forces from Afghanistan could come sooner than expected, the head of Nato has said as he conceded that the recent Taliban strategy of "green on blue" killings had been successful in sapping morale.

In an interview with the Guardian Nato's secretary-general, Anders Fogh Rasmussen, responded to pressure for a faster withdrawal from Afghanistan by stating that the options were being studied and should be clear within three months.

"From now until the end of 2014 you may see adaptation of our presence. Our troops can redeploy, take on other tasks, or even withdraw, or we can reduce the number of foreign troops," he said. "From now until the end of 2014 we will see announcements of redeployments, withdrawals or drawdown … If the security situation allows, I would not exclude the possibility that in certain areas you could accelerate the process."

Rasmussen admitted that the killings of almost 50 allied troops this year in "green on blue" attacks – Afghan security forces turning on their trainers and mentors – had damaged the relationship between the international forces and the Afghan police and military.

"There's no doubt insider attacks have undermined trust and confidence, absolutely," he said.

Nato aims to have an Afghan security force of 352,000 taking over responsibility for the country in just over two years when the US-led combat operations are scheduled to end.

Amid argument among analysts as to what has been behind the stream of "green on blue" attacks, Nato officers on the ground are reported to have ascribed them mainly to disgruntled and embittered Afghan security forces with grudges against their western mentors.

While Rasmussen conceded there may have been some such cases, Nato has clearly concluded from intelligence that the attacks have more to do with a Taliban strategy of infiltration of the Afghan security structures aimed at 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.

While it was allied soldiers who were being killed, Rasmussen said the Taliban campaign's ultimate target was to turn western public opinion against the war.

"Political leaders in the capitals of troop-contributing countries know very well that this is part of a tactic or strategy to also undermine public and political support at home … The real target is politicians, media, opinion-formers at home in partner nations and allied nations."

He added that some of the killings had been carried out by Taliban infiltrators disguised as Afghan police or soldiers.

"We have seen also where they were in Afghan uniforms though they are not members of the Afghan security force."

On the pace and phasing of withdrawal – the difficult task of pulling out more than 120,000 troops from forbidding and frequently hostile terrain – Rasmussen said a key moment would come later this year when General John Allen, the US overall commander of the operations, delivered a report with his military recommendations.

"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."

Rasmussen stressed that any accelerated rate of withdrawal should not be seen as "a race for the exits". The end of combat operations is to be followed from 2015 by a Nato-led training mission for the Afghan security forces, which will also require the continued deployment of fighting units or special forces, – "enablers" as they are called in military jargon.

"The core will be a training mission. Of course, we will have to ensure that our trainers can operate in a secure environment so we need capabilities to make sure that our trainers can operate," said the former Danish prime minister, who was appointed head of Nato in 2009.

Additionally, there will be further US forces remaining in Afghanistan under a bilateral "strategic partnership" deal struck between Washington and Kabul.

Amid mission creep fatigue across Europe, squeezed defence budgets and sweeping austerity cuts, it is not clear how many of the European Nato allies will contribute troops to the training mission. Defence ministers meet next week in Brussels to start planning the operations.