Relations between Tokyo and Beijing have soured following remarks by new Japanese Prime Minister Sanae Takaichi suggesting that Tokyo could intervene militarily in any attack on Taiwan.

Chinese J-15 fighter jets twice locked radar on Japanese fighter jets on Saturday, without causing damage or injuries, Japan's defence ministry said.

Fighter jets use their radar for fire control to identify targets as well as for search and rescue operations.

Defence Minister Shinjiro Koizumi said the incident was "dangerous and extremely regrettable", saying Japan had lodged "a strong protest" with China.

The Chinese navy said Tokyo's claim was "completely inconsistent with the facts" and told Japan to "immediately stop slandering and smearing".

China claims nearly all of the South China Sea, locking it into disputes with several Southeast Asian neighbours.

Beijing also claims self-ruled Taiwan as part of its territory and summoned Tokyo's ambassador following Takaichi's comments last month.

Tokyo is deepening cooperation with US allies in the Asia-Pacific region including Australia, whose Defence Minister Richard Marles is visiting Japan.

Marles said he was "deeply concerned by the actions of China in the last 24 hours", following a meeting with his Japanese counterpart.

He said Canberra had "stabilised" its relationship with China by "acting in a clear, consistent, calm and sensible way".

"We will seek to work together with China where we can, but we will disagree where we must," he said in response to a question about supply chains.

Marles also visited Mitsubishi's shipyard in Nagasaki, months after Canberra signed a major $6 billion deal to buy 11 advanced warships from the Japanese firm.