A Chinese professor of Japan's Asia University has become unreachable in China since February last year after he temporarily returned to his home country, sources familiar with bilateral relations said Sunday.

File photo shows Fan Yuntao, a Chinese professor of Japan's Asia University, in Yangzhou, China, in September 2018. (Kyodo) 

Fan Yuntao, 61, was contacted by Chinese authorities before he disappeared, the sources said, adding he may have been detained. The whereabouts of Hu Shiyun, a professor at Japan's Kobe Gakuin University, also became unknown in China, the college said in March.

Their disappearances have drawn attention as China has been strengthening its efforts to safeguard national security, leading to detentions of people, including foreign nationals, on suspicion of engaging in spying activities.

Beijing's intensification of crackdowns on foreigners and Chinese people who interact with other countries is expected to deepen the existing strains in Sino-Japanese ties, already frayed by several issues such as a territorial row.

In late February 2023, Fan returned to Shanghai. Before he went missing, he told people around him that he was asked to accompany authorities for interrogation, the sources said. He has a chronic illness that requires appropriate treatment.

Fan, an expert in international law and politics, had planned to be back in Japan by April last year, but his family living there has been still unable to establish contact with him. He has been placed on leave, and another teacher has assumed responsibility for his university classes.