As alliance leaders gathered in Lisbon for a summit intended to agree an exit strategy for the international military mission, Nato Secretary-General Anders Fogh Rasmussen stressed that as yet there were no "concrete plans" to withdraw troops. It is expected that Nato leaders and other troop contributing nations will sign up today to a plan for a phased handover of responsibility for security to Afghan forces. The transition process is due to begin next year and to be completed by 2014. Prime Minister David Cameron has said that British involvement in major combat operations will be over by 2015. Mr Rasmussen, however, stressed that while the transition plan offered a "realistic road map", it was still dependent on improvements within the Afghan security forces. "Our mission will end when the Afghans are capable to take responsibility themselves," he told the BBC Radio 4 Today programme. "We hope that they will be able to take such lead responsibility all over Afghanistan by the end of 2014. "But obviously this process must be condition-based. We have to make sure that the Afghans are actually capable to take responsibility before we hand over responsibility to the Afghans." Pressed on US plans to start bringing troops home next year, he said: "I'm not aware of concrete plans for withdrawal of troops. "On the contrary, I think all allies are prepared to stay committed as long as it takes to finish our job." Defence Secretary Liam Fox, who is attending the summit with Mr Cameron and Foreign Secretary William Hague, acknowledged that even after 2015, there would still be a non-combat role for British forces.