Japan has summoned the Chinese ambassador in Tokyo, after a Chinese frigate sailed near disputed islands in the East China Sea.
A foreign submarine was also spotted near the islands on Wednesday and Thursday, although its nationality was not clear, Japan’s military said.
Japan controls the uninhabited Senkaku islands, which China also claims under the name Diaoyu islands.
The tiny islands are a frequent flashpoint between the two countries.
They are prized because they are close to key shipping lanes, offer rich fishing grounds and lie near potential oil and gas reserves.
China regularly sends coastguard vessels near the islands – but sending a frigate, and possibly a submarine, suggests Beijing is becoming more assertive about its claims, the BBC’s East Asia editor Michael Bristow reports.
Japan said the large warship entered contiguous waters around the islands at 11:00 local time (02:00 GMT), and a submarine was also detected there, although neither entered Japanese territory.
According to UN conventions, contiguous zones are a band of water beside a country’s territorial seas, where the country can exercise some limited control.
Japan said it had summoned China’s ambassador and “issued a protest by expressing serious concerns”.
Meanwhile, Beijing’s foreign ministry spokesman Lu Kang said China had been conducting “surveillance over the activities of the Japanese side”, and said that the islands belonged to China.
Relations between Japan and China deteriorated in 2012 after Japan bought the disputed islands from a private owner.
In recent years, China has sent an increasing number of ships towards the islands, in what is seen as an attempt to test Japan’s resolve to defend them.