China has ramped up pressure on its neighbour since Japanese Prime Minister Sanae Takaichi suggested in November that Tokyo may react militarily to an attack on Taiwan, which Beijing has vowed to seize control of by force if necessary.

"Generally speaking, it is important that the overseas export of Japan's agricultural, forestry, and fishery products and foodstuffs is smooth," chief government spokesman Minoru Kihara said when asked about the reports.

He told a regular briefing that the same was true of rare-earth minerals, which are crucial for an array of tech products from smartphones to missiles.

"Furthermore, China's export control measures on rare earths and other materials have been ongoing for some time and are having a serious impact on the global supply chain," Kihara said.

"We will continue to monitor the situation closely and, in coordination with relevant countries, take necessary measures if required," he added.

Kyodo News reported that China-bound exports of Japanese sake and food items have been delayed, citing trade industry sources.

The "slow customs clearance" is believed to be part of China's reaction to Takaichi's comments about Taiwan, Kyodo said.

The Japanese embassy in Beijing has received enquiries from affected companies, and has urged China "to properly handle trade procedures and ensure transparency," the news agency reported, citing an unnamed source familiar with bilateral relations.

Separately the Wall Street Journal reported that China has begun choking off exports of rare earths and rare-earth magnets to Japan.

On Tuesday, China announced a broad ban on the export to Japan of "dual-use" goods with potential military applications.

China has since begun restricting exports to Japanese companies of scarce and expensive "heavy" rare earths, as well as the powerful magnets containing them, the Journal said, citing two exporters in China.

- Tensions rising -

China is by far the world's biggest supplier of rare earths and has long used its dominance for geopolitical leverage, including in its trade war with US President Donald Trump's administration.

An earlier spat in 2010 saw Japan move to lessen its dependence on Beijing for rare earths but more than 70 percent still come from China, according to Tokyo.

"China remains committed to safeguarding the stability and security of global industrial and supply chains," Chinese Commerce Ministry spokesperson He Yadong said Thursday.

"Civilian-use products will not be affected by this (ban on dual-use items). There is absolutely no need for parties engaged in normal civilian trade to be concerned," He told a regular briefing.

China also announced on Wednesday an anti-dumping probe into Japanese imports of dichlorosilane, a key chemical used in making semiconductors.

Last month, China conducted major military exercises simulating a blockade of Taiwan.

China has also warned its citizens against travel to Japan, and told Chinese people there to be careful.

In December J-15 jets from China's Liaoning aircraft carrier twice locked radar on Japanese aircraft in international waters near Okinawa, according to Japan.