The 10 aid workers shot dead in Afghanistan were part of International Assistance Mission, the group confirmed Monday.

Until last Thursday, none of the Christian non-profit's workers had ever been killed while on duty with the organisation – which has worked in Afghanistan since 1966.

"We want to pay tribute to each of our colleagues who died, to their commitment to serve the Afghan people," Dirk R Frans, executive director of IAM, said in a statement.

"Those who have known them and seen them at work can do nothing but pay the highest tribute to them."

The 10 workers were running the IAM Nuristan Eye Camp, work they had received permission from the Afghan government to do. They had just trekked 100 miles back through the Hindu Kush mountains, giving eye care to some of the poorest and most remote communities in Afghanistan, when they were led to a forest by gunmen, robbed, lined up and shot.

It is the largest massacre of aid workers in Afghanistan in years.

The victims include a Briton, six Americans, a German and two Afghans. They were as young as 24 and as old as 63. Two other Afghan workers who were part of the eye camp team survived. One was released after reciting Koranic verses and the other had left before the insurgents attacked, according to The New York Times.

The Taliban claimed responsibility, accusing the workers of spying and of proselytising, which is illegal in the south Asian country. IAM has refuted the allegations.

Though it is a Christian organisation, Frans said, "Our faith motivates and inspires us – but we do not proselytise. We abide by the laws of Afghanistan.

"We are signatures of the Conduct for the International Red Cross and Red Crescent Movement and NGOs Disaster Response Programmes, in other words, that, 'aid will not be used to further a particular political or religious standpoint.'

"But more than that, our record speaks for itself. IAM would not be invited back to villages if we were using aid as a cover for preaching," Frans maintained.

Family members paid tribute to British victim Dr Karen Woo on Sunday.

They described the 36-year-old doctor, who was to marry in two weeks, as someone who could "inspire others to give love and aid rather than perpetuate hate and violence".

"Her commitment was to make whatever difference she could," they added.

"She was a true hero. While scared, she never let that prevent her from doing things she had to do.

"Karen, you were an inspiration to everyone you met. You combined brains and beauty, intelligence, drive and kookiness in equal measure."

US Secretary of State Hillary Clinton on Sunday condemned what she called a "senseless act".

"We also condemn the Taliban’s transparent attempt to justify the unjustifiable by making false accusations about their activities in Afghanistan," she stated.

"Terror has no religion," she said, "and these acts are rejected by people all over the world, including by Muslims here in the United States.

"The murdered medical aid workers, as well as the volunteers from many nations and the international coalition working to establish stability in Afghanistan, represent exactly what the Taliban stands against: a future of peace, freedom, opportunity, and openness, where all Afghans can live and work together in harmony, free from terror."

Clinton noted that the workers were not being paid for their services and had travelled to Afghanistan because they wanted to help people in need.

"We are heartbroken by the loss of these heroic, generous people," she added.

IAM is an international charitable organisation, serving the people of Afghanistan, through capacity building in the sectors of health and economic development. It has about 500 Afghan workers and 50 international ones.

In June, an inspection team of the Afghan Ministry of Economy visited the Christian group to get an overview of its history and current activities. The team informed IAM that there were other teams doing inspection visits to the some 350 international NGOs currently in Afghanistan.

They asked where the group's funding came from and whether any of the funding came from the money pledged by the international community to Afghanistan.

"They were pleased with the answer that our funding did not, and that all our funding was 'extra,'" IAM stated in an earlier announcement. "They were also pleased that none of our expats get any salary from IAM."

The recent murders have devastated the organisation but the group is not thinking of withdrawing from Afghanistan as things stand right now.

"Our NGO has worked here for well over four decades," Frans noted.

"And we remember that there were times when security was much worse than it is now. IAM works in Afghanistan as the guest of the people and the government.

"As long as we are welcome here, we will, God-willing, continue to stay and serve the Afghan people."

According to Frans, the Ministry of Interior of the Islamic Republic of Afghanistan, the FBI and others are involved in the investigation to find out who committed the massacre.

The other victims of the massacre were Cheryl Beckett, Brian Carderelli, Dr Tom Grams, Glen Lapp, Dr Tom Little, Dan Terry, Daniela Beyer, Mahram Ali and Jawed.

Little was "the driving force behind much of what has been achieved in eye care in Afghanistan," Frans stated. "He is irreplaceable."