Japan and China will mark 50 years of diplomatic relations this month with little public celebration, as increasing friction over territorial rivalries and military spending frays ties.

The world's second and third largest economies are key trading partners and just a few years ago seemed poised for a diplomatic blossoming, with plans for a state visit by Chinese President Xi Jinping.

Since then, ties have soured significantly as Beijing bolsters its military, projects power regionally and beyond, and takes a harder line on disputed territory.

In recent months, Chinese missiles are believed to have fallen within Japan's exclusive economic zone, and Tokyo has protested what it calls growing aerial and maritime violations.

Japan also regularly complains about Chinese activity around the disputed Tokyo-controlled Senkaku islands, which Beijing claims and calls the Diaoyus.

"Chinese ships are loitering for dozens of days in the East China Sea, while an artificial island and a base have already been built in the South China Sea," said Kenichiro Sasae, head of the Japan Institute of International Affairs.

"That makes us wonder — how much further will China's ambition go in terms of naval power?" added Sasae, a former ambassador to Washington and vice minister of foreign affairs.

The war in Ukraine has only deepened the divide, with Japan backing Western allies opposed to Russia's invasion while Beijing avoids criticizing Moscow.

And the conflict has refocused attention on whether China might try to forcibly reunite Taiwan with the mainland, prompting Japan's Prime Minister Fumio Kishida to warn the Ukraine situation "could be East Asia tomorrow".

'Anxious about China'

Beijing and Tokyo normalized relations in a joint communique on September 29, 1972, that officially ended their state of war and saw Japan drop its recognition of Taiwan.

Economic ties grew quickly and steadily, but political relations have been more unstable, weathering a series of crises, including over the disputed Senkaku-Diaoyu islands.