Japan's consular offices in China were notified last month that one Japanese citizen was apprehended in the northeastern port city of Dalian on suspicion of "smuggling goods subject to a national export and import ban", spokesperson Minoru Kihara told a briefing.

A week later, Japanese authorities were separately informed that another Japanese national was detained on the same allegation, Kihara added.

"These cases remain under investigation," he said, adding the two individuals appear to be in good health and that the government will "take appropriate measures from the standpoint of their protection".

Beijing's foreign ministry said on Wednesday that the two Japanese citizens had been detained "for violating Chinese law", without giving details.

"We would like to emphasise that Japan should educate and remind its citizens and businesses in China to abide by Chinese laws and regulations," spokesperson Guo Jiakun told a regular news briefing.

The relationship between Japan and China worsened after Prime Minister Sanae Takaichi suggested in November that Japan could intervene militarily if China attempted to seize Taiwan, the self-ruled island claimed by Beijing.

Under Takaichi, Japan has quickened its pivot towards a more proactive defence policy, further shaking off -- with US encouragement -- its pacifist outlook in place since the end of World War II.