All 21 members of what is thought to be a Christian group from Japan detained in China since May have now returned home, according to a Foreign Ministry source.

Many of the group members, all Japanese citizens, are likely to have been deported, the source said Monday.

There were no further details immediately available, but Chinese authorities have been intensifying efforts to crack down on missionary work, labeling it an illegal activity.

According to the source, the 21 Japanese were detained sometime between May 5 and 15 across a wide range of areas, including in the southwestern city of Chongqing and in Hebei province, northeastern China.

Five had returned to Japan by May 25 while the other 16 remained in detention, according to a source close to Japan-China relations.

China also detained 19 Japanese nationals affiliated with a Christian group in November in the southeastern province of Guangdong before deporting all of them, the source said.