Tokyo lodged an official protest with Beijing Wednesday after Chinese ships cruised around disputed islands, days before a historic summit between the leaders of the Asian rivals.
Japan's top government spokesman Yoshihide Suga called the intrusion "extremely regrettable" but said Tokyo was nonetheless hoping to "take Japan-China relations to the next level".
The complaint, which is only the latest in a string of similar protests, comes before Prime Minister Shinzo Abe heads to China next week for talks with President Xi Jinping.
The trip will be the first official visit to China by a Japanese leader in seven years, as Asia's two biggest economies take gradual steps to improve their tense relations, marred by bitter memories of the World War II.
Abe has only been to China for international events in recent years, with relations between the two nations deteriorating from 2012 when Tokyo "nationalised" some of the disputed islets.
Since then, the two top Asian economies have taken gradual steps to mend fences but relations remain tense.
China routinely dispatches its coastguard ships, and sometimes military vessels, to waters around the islands, which are called Senkaku by Japan and the Diaoyu islands by China.
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