Tokyo denounced Beijing on Thursday after a Chinese frigate entered a contiguous zone just outside Japanese territorial waters earlier in the day near the disputed Senkaku Islands in the East China Sea.
The move comes amid mounting international pressure on China over its growing maritime assertiveness.
"Beijing's action of sending a military ship for the first time would further escalate tensions unilaterally and we are gravely concerned about it," Chief Cabinet Secretary Yoshihide Suga told a news conference.
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