For the second time in two days, Japan has protested to Beijing over Chinese coastguard ships and fishing vessels sailing into waters close to disputed islands in the East China Sea.
The two countries are locked in a long-running dispute over the uninhabited islets, known as the Senkaku in Japan and the Diaoyu in China.
Japan's foreign ministry said that seven Chinese coastguard ships, including four that appeared to be armed, and about 230 fishing vessels sailed on Saturday close to what Tokyo considers its territorial waters.
In a statement, the ministry called for the immediate withdrawal of the vessels.
"Japan can never accept activities by [Chinese] official vessels near the Senkaku islands because it will unilaterally escalate the situation and raise tensions in the area."
Beijing also claims the islands and occasionally sends its coastguard vessels near them. However, it is rare for so many Chinese fishing vessels to be spotted in the area.
Saturday's protest came a day after Japanese vice foreign minister Shinsuke Sugiyama summoned Cheng Yonghua, China's ambassador to Tokyo, to protest over intrusions into its territorial waters by Chinese coastguard and fishing vessels on Friday afternoon.
Tensions over the islands have seriously harmed bilateral relations.
The two sides have gradually taken steps to ease tensions through dialogue, but the fundamental divide over the islands remains unresolved and tensions occasionally flare up.
Japan also lodged a protest in June after it said a Chinese navy frigate sailed close to territorial waters near the islands for the first time.
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On Friday, Beijing accused Japan's new defence minister, Tomomi Inada, of recklessly misrepresenting history after she declined to say whether Japanese troops massacred civilians in China during World War Two.
The latest incident comes amid heightened tensions in Asia, less than a month after an arbitration court in The Hague invalidated China's sweeping claims in the disputed South China Sea, in a case brought by the Philippines.
But China has refused to recognise the court ruling, reacting angrily to calls by Western countries and Japan for the decision to be adhered to.
Beijing has repeatedly blamed the US for stoking tension through its military patrols in the South China Sea, a strategic waterway through which more than $5 trillion of trade moves annually.
'Show of strength'
Michael Cucek, a Japan analyst and professor at Temple University, said Beijing's latest move in the East China Sea was a show of strength by China.
"That's a traditional way of the Chinese sending a wake-up call, is to engage in some kind of very aggressive act," he told Al Jazeera.
"China has [also] spoken about an air defence zone over the region," Cucek added, noting that Beijing has "a vast superiority in air power" over Japan.
"So from the Chinese point of view, it wants to break up all these different disputes and keep them small and controllable; and one way of doing that is by reminding the Japanese that any cooperation ... with the United States, the 'hegemon' of that region, is going to cost them."
Source: Al Jazeera and agencies