Eight foreigners and two Afghans have been found shot dead next to abandoned vehicles in the north-eastern Afghan province of Badakhshan, officials say.
The foreigners were six Americans, one Briton and a German, who worked for an international charity providing eye care and medical help.
The vehicles were found a day after contact was lost with the group.
Local police said robbery might have been the motive. However, the Taliban have said they were behind the attack.
Zabihullah Mujahid, a Taliban spokesman, said Bibles translated into Dari had been found.
"Yesterday at around 0800 (0330 GMT), one of our patrols confronted a group of foreigners. They were Christian missionaries and we killed them all," he told the AFP news agency.
He later told the Associated Press they were "spying for the Americans".
The team comprised five American men and three women - an American, a German and a Briton, along with four Afghans. Two of the foreigners worked for the International Assistance Mission (IAM), two were former IAM workers and the others were affiliated to other organisations. One Afghan was spared, and another Afghan was not with the group at the time of the attack.
IAM executive director Dirk Frans denied they were missionaries.
"That is a lie. That is not true at all. IAM is a Christian organisation, we have always been that," he told the BBC.
"We have worked in Afghanistan since 1966 - under the king, the communists, the Russians, the Mujahideen and the Taliban. They have known us as a Christian agency, but we certainly do not distribute Bibles."
An IAM spokesman earlier said it was still awaiting formal identification of the victims, but that their families had been informed.
Mr Frans said one of those killed was Tom Little, an optometrist from New York who had been working in Afghanistan for more than 30 years.
The charity Bridge Afghanistan, which was working in conjunction with IAM, named the British aid worker as Dr Karen Woo.
The bodies are expected to be returned to Kabul over the weekend.
The two dead Afghans were interpreters.
The US embassy in Kabul said it had reason to believe that several American citizens were among the deceased.
"We cannot confirm any details at this point, but are actively working with local authorities and others to learn more about the identities and nationalities of these individuals," it said in a statement.
The UK's Foreign and Commonwealth Office told the BBC it, too, was aware of the reports that bodies had been found in northern Afghanistan and was "urgently looking into this".
Badakhshan, a mainly ethnic Tajik region bordering Tajikistan, is one of the few Afghan provinces not to have been controlled by the Taliban before the US-led invasion of 2001.
The BBC's Quentin Sommerville in Kabul says it has long been regarded as a safe area, although locals have complained about the growing threat from insurgents.
The IAM said its eye camp team had been working for the past two-and-a-half weeks in the neighbouring province of Nuristan at the invitation of communities there.
The group's leaders had decades of experience working in Afghanistan, Mr Frans said.
The team was returning to Kabul via Badakhshan because they thought that would be the safest route, he added.
IAM said it lost contact with them on Wednesday evening, after which their convoy is thought to have come under attack.
Our correspondent says attacks on humanitarian workers are unusual in Afghanistan, and their vehicles would probably have been marked.
'Nothing left behind'
Badakhshan's police chief, Gen Agha Noor Kemtuz, told the Associated Press that the victims had been found dead in the district of Kuran wa Munjan, and had been stripped of their possessions.
"Nothing was left behind," he said.
Gen Kemtuz said a third Afghan man who was travelling with the group had survived.
"He told me he was shouting, reciting the holy Koran and saying: 'I am a Muslim. Don't kill me'," he added.
The fourth Afghan team member had taken a different route as he had family in Jalalabad.
In a statement published on its website on Saturday, IAM said: "At this stage we do not have many details but our thoughts and prayers are with the families and friends of those who are presumed killed."
"If these reports are confirmed we object to this senseless killing of people who have done nothing but serve the poor. Some of the foreigners have worked alongside the Afghan people for decades."
"We hope it will not stop our work that benefits over a quarter of a million Afghans each year," it added.