TOKYO (AP) — Japan and Australia urged calm on Sunday after Chinese military aircraft locked radar on Japanese fighter jets, a month after the Japanese leader’s recent remarks on Taiwan that stirred tensions between Tokyo and Beijing.

Japan is quickly transforming its southernmost Ryukyu islands into a heavily fortified defense zone, sparking concern and curiosity across the region. The move comes as relations between Japan and China reach levels of tension not seen in over a decade.

Chinese officials took issue with Japan’s claim. Wang Xuemeng, a spokesman for the Chinese Navy, said in a statement that the Liaoning was carrying out “routine carrier-based fighter jet flight training” that had been announced in advance.

It is the first time the U.S. has criticized China for aiming radars at Japanese military aircraft as the diplomatic spat between Beijing and Tokyo shows no sign of easing.

Kareem El, a civilian from Washington, D.C., visiting Okinawa, was held by U.S. military police for declining to provide identification.

American strategic bombers trained alongside Japanese aircraft in what Washington and Tokyo described as a demonstration of their resolve following recent military tensions between Japan and China this week.

While diplomatic tensions between the two countries are not new, both have little to gain from the current dispute subsiding.

Representatives from the U.S. Army Corps of Engineers – Japan Engineer District (USACE JED) attended the Society of American Military

Japan plans to deploy medium-range surface-to-air missiles on Yonaguni – making it a frontline of defence in the event of a Chinese attack on Taiwan – as part of its broader military build-up across the southern island chain.

The Japanese defense ministry on Saturday reportedly claimed spotting the aircraft carrier Liaoning of the Chinese People’s Liberation Army (PLA) Navy near Japan. A Chinese military affairs expert said that Chinese aircraft carriers’ far seas exercises in the Pacific Ocean have become routine,