Chinese authorities detained 35 Japanese nationals earlier this month suspected of fraud in Fujian province, a Japanese government source said Wednesday.

The suspects are allegedly involved in phone scams targeting victims in Japan, who are tricked into making bank transfers of large sums of money, a source involved in Japan-China relations said.

Chinese authorities notified Japan on July 3 that the nationals had been taken into custody, according to the Japanese government source. Criminal custody is often a step taken by Chinese police before making formal arrests.

Japan and China have been negotiating an extradition treaty since 2010 but have yet to seal a pact. Japan will need to negotiate through diplomatic channels if it wants the suspects extradited.

China has detained at least 12 Japanese nationals since 2015 over allegations including espionage, with the Japanese government expressing concern over a lack of transparency in Beijing's standards for detention.