China has detained two Japanese citizens on suspicion of smuggling banned products, the Japanese government said on Wednesday, adding to rising tensions between the two countries.

The Japanese nationals were detained in May in the northeastern Chinese city of Dalian, according to a statement by Japan’s government. Chinese officials have indicated that they are being held on “on suspicion of violating the law and smuggling goods that are prohibited from being imported or exported,” Minoru Kihara, a top Japanese cabinet official, said at a news conference in Tokyo on Wednesday.

He added that Japan is working to “respond appropriately from the perspective of protecting Japanese nationals.”

Japanese news outlets reported that the detention appeared to be related to the export of rare earths — minerals that are used to make a range of essential technologies — by a Japanese firm operating in the mainland. But Japanese officials declined to provide further details, saying the matter was under investigation.

The Chinese foreign ministry did not immediately respond to a request for comment.

China has severely restricted the export of rare earths to Japan in recent months, part of its campaign to punish Tokyo over a comment in November by Prime Minister Sanae Takaichi, a China hawk. She told the Japanese Parliament that a hypothetical Chinese attack on Taiwan could incite a military response from Tokyo.

China, which considers Taiwan, a self-governed democracy, part of its territory, responded by unleashing a wave of political and economic reprisals against Japan.

China dominates the global supply of rare earths. It cut exports of them to Japan by more than 80 percent in March and April compared with last year, according to Nikkei, a Japanese media company. That has forced Japanese companies to scramble for alternatives.

Ms. Takaichi raised the issue of China’s stranglehold on rare earths during the recent Group of 7 summit meeting in France. She pitched a plan to create a joint stockpile of critical minerals among the seven nations.

As part of its efforts to punish Japan, Beijing has restricted the import of Japanese seafood and discouraged Chinese citizens from visiting Japan. China has also ramped up its presence in the seas surrounding Japan. Beijing recently led coast guard patrols in the waters between Japan and Taiwan.

Japan said on Wednesday that a Chinese mobile drilling vessel has recently anchored in the East China Sea. Japanese officials have expressed concern about China’s development of gas fields in those waters, saying Beijing has done so without input from Japan.

At the news conference on Wednesday, Mr. Kihara denounced the move.

“It is extremely regrettable that, despite repeated protests from Japan, China continues to engage in unilateral development activities in these waters and attempts to create a fait accompli,” he said.

Pei-Lin Wu contributed reporting from Taipei, Taiwan.