Britain's top military leader said on Sunday that it was not possible to militarily defeat "Islamic extremism", and that defeating al-Qaida and the Taliban in Afghanistan depended on balancing support for the Afghan government and development aid with military actions.

The new British Chief of Defense Staff, General David Richards, who took up his post only a few weeks ago, said al-Qaida was an extreme form of Islam which "it was not possible to defeat".

General Richards agreed that there was always likely to be an " extremist" element to Islam, and that its threat came not in its existence but "how it manifests itself and can we contain that manifestation, and clearly al Qaida is an unacceptable manifestation of it".

Speaking in a BBC TV interview General Richards backed the 2015 deadline set in the summer by prime minister David Cameron for the withdrawal of all British combat troops from Afghanistan.

The key to hitting the deadline "was whether the armed forces would be given the resources... so that our renewed focus could be given effect on the ground, and that has happened," he said, adding that withdrawal "could start next year, we are sort of on track."

Richards agreed that mending Afghanistan "will take 30 or 40 years", and said British troops would remain in a combat role for four to five years, "but progressively less so as the Afghan police and army grow in capability".

He added that foreign intervention would continue to be a reality in Afghanistan for a long time, "Everyone is clear that we will have to remain a lot longer than that to ensure that we consolidate on all that hard work and the plans are now in place."

He said that the NATO summit in Lisbon in a week's time "will make that rather clearer than it has been to date".

General Richards called for talks with the Taliban to find a peaceful solution in Afghanistan. "In every insurgency in history there has been a point where you negotiate with what is left of the opposition and in the case of the Taliban there are people we know who would at some point wish to negotiate," he said.

"We should not worry about it, it is part and parcel of this kind of operation," he added.