Gift of the Givers, the charity that organized the tour, said that China had detained some of the group because of suspicions of alleged links to terror groups and for watching propaganda videos in their hotel rooms in a city in northern China. It stressed they had no terror links or criminal record.
"Twenty foreigners were suspected of violating laws and committing crimes in Ordos, Inner Mongolia, on July 11," China's foreign ministry said in a written statement to CNN.
The ministry did not address CNN's question on the reason for their detention.
"The case is under investigation. China will guarantee the legitimate rights of relevant people in accordance with law, and will continue to provide necessary convenience and assistance to counselor officials of foreign embassies, " the ministry added.
In a Facebook post, Gift of the Givers, a large disaster relief organization based in South Africa, said the 20 foreigners included 10 South Africans, nine Britons and one Indian.
The post said the group was on a 47-day journey to explore ancient China --- until they were suddenly stopped by police at the Ordos airport last Friday, July 10.
"They were detained without charge, with no access to any communication nor to legal representation," the post said.
"It was only on Sunday, 48 hours later, when the tour operator realized that something was amiss and made the trip to Ordos, that the first information on the detained citizens came to light."
"The Chinese, now trying to find reasons for the detention suggested that some members were linked to a terror group, to a banned organization, to watching propaganda videos in their hotel room."
Quoting family members, the post called on the Chinese government to release immediately the nine people who remain in detention.
"These individuals have no terror links, no criminal record in their country," it added.
The British embassy in Beijing confirmed that nine British nationals - as well as two dual British-South African nationals - were among those detained, adding that six had been deported.
"Consular staff have visited the group to provide assistance and we are liaising with Chinese authorities," said a spokesman with the British Foreign Office. It added that it is seeking further clarification from Beijing on the reason of their detention.
A spokesperson for the South African Department of International Relations and Cooperation (DIRCO) said five South Africans were headed home and five other remained in China.