Mr Holbrooke said, however, that the heart of the threat facing Nato in Afghanistan comes from western Pakistan, where the Taleban have much support.

BBC: Envoy damns US Afghan drug effort

He described Pakistan's border region with the country, where he met troops fighting the Taleban, as one end of a "chain of terror" that could stretch to Britain if more was not done to tackle the threat of al-Qaeda.

BBC: Pact targets Pakistan terror link

The US has specifically threatened military action against Afghanistan, where he lives, if the country's Taleban leaders do not hand him over.

BBC: Q&A: Military options