Japan scrambled fighter planes on Wednesday after Chinese boats and military aircraft approached a disputed set of islands in the East China Sea.
Four Chinese ships were spotted in the waters around the Senkaku islands, which Beijing claims and calls Diayou.
A Chinese military aircraft also flew through the airspace of Okinawa prefecture's main island and the smaller Miyako island in southern Japan and then back along the same route.
The move sparked alarm in Tokyo.
"I believe this indicates China's move toward further maritime expansion," Japanese Defense Minister Itsunori Onodera told reporters.
The Chinese government has not commented on the incident.
The tiny uninhabited islets are to the west of Okinawa and about equidistant from China and Japan.
More from GlobalPost: Is the world ready for a muscular Japan?
Until September, they were owned by a private Japanese citizen but were sold in September to the Japanese government, sparking tension with China that claims the rocky territories.
It is believed that the areas around the islands may contain oil and gas reserves. The islands also fall within rich fishing grounds.
Japanese and Chinese patrol ships and airplanes have shadowed each other around the islands since they were sold in September.
Japanese Prime Minister Shinzo Abe has taken a firm stance on the islands — a position that could harden after last Sunday's electoral victory despite calls for dialogue.
An ascendant and more aggressive China has sparked debate in Japan about bolstering its military capacity despite constitutional impediments.
Japan has been constitutionally barred from creating a standing army since 1947.