A Chinese navy frigate has locked its weapon-targeting radar on a Japanese ship, Tokyo says, amid mounting tensions over a territorial row.
Japanese Defence Minister Itsunori Onodera said the incident happened on 30 January near islands claimed by both nations in the East China Sea.
He said this had prompted Tokyo to lodge a formal protest with Beijing.
The row, over islands known as Senkaku in Japan and Diaoyu in China, has escalated in recent months.
Taiwan also claims the island chain (known as Diaoyutai in Taipei), which is controlled by Japan.
Last week, tensions between Tokyo and Beijing appeared to be easing after a Japanese delegation met senior Chinese leaders and both sides later expressed hopes that relations could improve, the BBC's Rupert Wingfield-Hayes in Tokyo reports.
But on Monday China sent patrol ships back in to the disputed waters around the islands, our correspondent adds.