In Lisbon, they talk. In Afghanistan, they die.

Christopher Davies, 22, was the 100th British serviceman to die this year in a war that Nato's leaders – gathered today for a crucial summit – have no idea how to win.

Christopher Davies, a guardsman with the 1st Battalion Irish Guards, has been named as the 100th member of Britain's armed forces fighting in Afghanistan to die this year.

The 22-year-old's death was given extra poignancy yesterday as world leaders gathered to formulate an exit strategy from the bloody and intractable campaign. It has now claimed the lives of 345 British servicemen and women since it began in 2001.

Guardsman Davies, from St Helens, Merseyside, died after being ambushed and shot by insurgents while on patrol in the Nahr-e Saraj district, Helmand Province, on Wednesday. He was the devoted father of one young daughter, Lucy. Lance Corporal Bryan Philips, one of his colleagues, said he "had so many pictures of his daughter and talked about her constantly".

His family remains entwined in the fortunes of the Afghanistan mission. He had been serving in the Irish Guards alongside his 21-year-old brother, John Davies.

In a statement issued yesterday, Guardsman Davies' family described him as a "cracking lad" who had a keen imagination, loved joking with friends and often entertained them by singing karaoke. He had joined the Army in January last year.

"Christopher had always wanted to be in the Army ever since he was at school," his family said. "His friends in the Army have told us that whenever they felt down he would cheer them up, often by singing. He had an invincible personality and we will miss him so much. There is a big hole in our lives." His death served as the most urgent of reminders to David Cameron and his delegation at the Nato summit in Lisbon that, as they searched for an end to the mission with foreign premiers, aides and military chiefs, families back home were counting down the hours until the safe return of their loved ones. More civilian and military casualties were recorded this year than any other since the conflict began in 2001. Frustration at the stagnation of the nine-year-old war will boil over on to the streets of London today, as thousands of anti-war protesters gather to demand the return of British troops.

The Nato summit, which began in Lisbon yesterday, is regarded as one of the most important in the organisation's 61-year history. It is due to set out the road map for withdrawal from the Afghan war. But yesterday there appeared to be differing opinions within the alliance on how that crucial endgame will be played out.

Mr Cameron insisted yesterday that British troops will end their combat role by 2015 with the Afghan government taking over responsibility for security. But in Washington, the Pentagon press secretary, Geoff Morrell, stressed that the timeline was "an aspirational goal" and there was no guarantee that a large-scale withdrawal of Western forces would take place by that date. And Nato's Secretary-General Anders Fogh Rasmussen stressed that, as yet, there were no "concrete plans" to withdraw troops from next July as Barack Obama had indicated when authorising the surge of tens of thousands of reinforcements.

Nato leaders will today present a communiqué stating that there will be a phased handover of responsibility for security to Afghan forces. 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 said. "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."

Asked about 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."

One senior Nato official with extensive experience of Afghanistan warned that while the US troop surge had had an effect, progress was still not "irreversible". "It is a conditions-based process, not a calendar-driven process," he said. "We are not indulging in a lot of happy talk about the security situation in Afghanistan. We have made real progress but that progress is not yet irreversible. What we have done is to regain the initiative."

Liam Fox, the British Defence Secretary, acknowledged that the Afghan government would still require international "help and advice" after 2015, and it was too soon to say what proportion of the current 10,000-strong British force would be needed after that date. He said: "If you look at what is happening in Iraq there are still high levels of violence there, we are not going to suddenly transition into a safe, secure and peaceful Afghanistan overnight."

Mr Fox added that British forces may be repositioned outside Helmand. He said: "As the security situation improves we will change the number of forces. "

The race will now be on to ensure Afghan troops can take up the slack, something that many analysts question. It was in this cause that Guardsman Davies died, one of 654 Nato troops to give their lives this year. He was struck by small arms fire as he partnered a group from the Afghan National Army to the east of patrol base Hazrat.

"His sacrifice will not be forgotten," Mr Fox said.