Sri Lanka, April 23 -- 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.

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 intensificati...