FILE PHOTO: Taliban flags fly at the airport in Kabul, Afghanistan, on Sept. 9, 2021.
The International Assistance Mission (IAM), international NGO, confirmed its staffers were picked up from its office in Ghor province, central Afghanistan, and taken to the capital Kabul.
Security and intelligence forces had been observing the group for some time, Abdul Wahid Hamas Ghori, a government spokesman for the province, told AFP.
"Documents and audios were obtained that showed they were inviting people to join Christianity," he said, without providing further details.
He said 21 people were arrested, including an American woman.
IAM said in a statement earlier that 18 people, including a "foreigner", were being held and that it had no information about the nature of the allegations.
The American woman and two Afghan staff were the first to be detained on September 3, followed by 15 more Afghan employees on Wednesday.
"Should any charges be lodged against our organisation or any individual staff member, we will independently review any evidence presented," it added.
IAM's website says the organisation is founded on Christian values, but that it does not provide aid according to political or religious belief.
"We value and respect local customs and cultures," the Swiss-registered group said in a statement on Saturday.
IAM has operated in Afghanistan since 1966 -- through previous royal, communist and Taliban governments -- when it specialised in eye care, later branching out into other areas of health and education.
In 2010, 10 IAM medics, including eight foreigners, were shot dead in an attack in remote northern Afghanistan.
At the time, competing theories emerged over the motive for the attack, with police saying it was likely a robbery.
However, two militant groups claimed responsibility, including Taliban leaders who said the medics were Christian missionaries and accused them of working as military spies.
Dozens of foreigners -- including several Westerners -- have been detained by the Taliban authorities since the group's return to power in August 2021.
The Taliban rulers have imposed sweeping restrictions on the population they say are in line with their strict interpretation of Islamic Sharia law -- including barring women from working for NGOs and the United Nations.
Teenage girls and women are also banned from schools and universities, and excluded from many other formers of public social life.