Chinese authorities detained 21 Japanese nationals in the southwestern city of Chongqing and elsewhere this month, a source close to Japanese-Chinese relations said Friday.

The Japanese are said to have been members of a Christian group and their detention may have been part of efforts by authorities to suppress missionary work.

The 21 were detained between May 5 and 15 in Chongqing and in five provinces — Hebei, Henan, Guizhou, Shanxi and Liaoning — as well as the autonomous region of Ningxia, according to the source.

Of them, three have already been expelled from China, while the rest are likely to remain in detention.