Gayle Williams worked with the poorest and most unfortunate of the children in Afghanistan, young boys and girls who had lost limbs to landmines and bombs. She was dedicated to her task of teaching them the basic skills needed to survive in a harsh and violent land. Yesterday the 34-year-old British woman was murdered while walking along a quiet, tree-lined street in Kabul on her way to work.

She believed in living among the people she served, staying in a modest private house, shunning an armed escort in favour of using her own two feet. She made an easy target for the two gunmen who had been lying in wait for her. One of the men got off the motorcycle, walked up to Ms Williams and opened fire at close range. She was already dead by the time her killers had weaved their way through the crowd at Kart-e-Char, leaving her corpse in a pool of blood.

"The men on the motorcycle stopped in front of the lady. They took out a gun and shot her on the spot. Then they rode off," said Daolad Khan, who was working on a building site directly opposite the murder scene. Mohammed Gul, a shopkeeper, added: "They knew what they were doing, they knew she would be there. She was hit many times on the chest and the body, no one could have lived after such an attack."

Ms Williams had been in the country for two and a half years, working for a charity called Serve Afghanistan, which helps disabled children and adults to learn to live with their handicaps. A Taliban spokesman, Zabiullah Mujahid, declared that she had been executed "because she was working for an organisation which was preaching Christianity in Afghanistan". Converting from Islam to Christianity is a capital offence in Afghanistan but friends and colleagues of Ms Williams stressed that while the organisation she worked for was Christian in its beliefs, she was extremely careful not to try to convert Afghans.

Mike Lythe, the head of the charity and a friend of Ms Williams, said: "This is a tragedy. She was working with disabled children, blind and deaf children, children hurt and maimed by the fighting. She knew the dangers, but Afghanistan is where she wanted to be."

Her mother, Pat Williams, who lives in London, and sister Karen, who has settled in Johannesburg, received the news of the death yesterday afternoon and were considering flying to Kabul. "Gayle was serving a people that she loved, and felt God called her to be there for such a time as this," her mother said last night.

"We know her life was blessed and she was a blessing to those around her," she added. "No one could have asked for a more humble daughter with a more loving heart. She died doing what she felt the Lord had called her [to] and she is definitely with him."

In one of her newsletters to friends and family, Ms Williams had written: "God has an amazing plan for this country and even though things so often look hopeless we can focus on Him knowing that He is able to see His plans and purposes fulfilled in Afghanistan."

Six months ago, Ms Williams had been pulled out of Kandahar and recalled to Kabul, after the abduction and murder of an Afghan colleague. In her newsletter, she explained that she and other charity workers were being watched: "We have been told that our office was under surveillance but no one could be held or charged due to insufficient evidence". On the same day United Nations security officials warned that armed men had been moving into Kandahar with the aim of kidnapping foreigners, and the decision was taken to evacuate Serve staff to Kabul.

Kandahar, the birthplace of the Taliban, has seen a series of bombings and shootings and at the time the Afghan capital was regarded as a place of relative safety. Since then Kabul has experienced an increase in violence. Ms Williams abided by safety instructions issued to aid workers, varying her route and clothing, leaving home at different times. But this did not save her from the gunmen and the murder added to fears among expatriates who are already feeling beleaguered in an Afghan capital being encircled by a resurgent Taliban.

Aid workers, in particular, have become targets. Three women – a British-Canadian, a Canadian and a Trinidadian-American – along with their Afghan drivers were killed outside Kabul two months ago. In another sign of unravelling security, Houmayun Assefy, a grandson of the Afghan king, Zahir Shah, was kidnapped in the city centre on his way home. Ms Williams' death also came on the day that a suicide bomber killed two German soldiers and five children in an attack in Kunduz in the north of the country, another area previously deemed to have been relatively safe.

Douglas Alexander, the International Development Secretary, condemned the murder yesterday. "Her killing was a callous and cowardly act by people who would take Afghanistan back to the dark days of the Taliban tyranny which scarred the country for so long," he said.

But for many in Afghanistan, the dark days are already back, with Taliban attacks happening with increasing frequency and in ever closer proximity to the capital Kabul. Nato commanders privately admit that a lack of cohesion in Western strategy has bolstered the insurgents. And yesterday General John Craddock, the head of Nato forces in Europe, warned that the West was "wavering". "That impedes operational progress and brings into question the relevance of the alliance here in the 21st century." General Sir David Richards, the new head of the British Army, believes that a "surge" of around 30,000 troops is needed to counter the Taliban.

Western politicians and military commanders have repeatedly stated, however, that a purely military victory in Afghanistan is not feasible, and that reconstruction and development of the country are vital components of a settlement. The prospect of this taking place, however, would be greatly undermined if the departure of aid groups from the country accelerates following Ms Williams' death.

"Obviously it makes aid workers very worried about staying in Afghanistan," said Mr Lythe of Serve. "I think other attacks may take place. It seems to be the policy of the Taliban to destabilise the country by trying to knock out aid work and drive out NGOs."

Ms Williams grew up in the town of Empangeni in KwaZulu Natal before moving to England with her mother. After attending secondary school in London, she returned to South Africa and studied biokinetics and occupational therapy at Zululand University. She subsequently returned to London where she worked with disabled and deprived children.

An avid cyclist and canoeist, she had always had a fascination with Afghanistan and was determined to work there. The pressure of working under constant pressure, however, was taking its toll. Mr Lythe recalled: "A group of us went on a walking holiday to Tajikistan recently and Gayle told us how wonderful it was to be able to relax, not to worry about things like keeping herself covered up in public. But she knew that it was a holiday, she needed to go back to her work. She was passionate about her work."