BEIJING: Japan on Thursday lodged a protest with China after four Chinese coastguard vessels entered what Tokyo considers its territorial waters near disputed East China Sea islets and a drone-like object flew near one ship, the Japanese government said.

It was the first such flight near the islands witnessed by Japanese officials, although Thursday's incident takes to 13 the number of intrusions this year by Chinese coastguard ships in the contested waters, Japan's coastguard said.

Japan and China have long been at loggerheads over the tiny, uninhabited islands, called the Senkaku in Japan and Diaoyu in China. They are controlled by Japan but claimed also by China.

Kenji Kanasugi, director-general of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, protested to the Chinese embassy in Tokyo by telephone.

"The Senkaku islands are Japan's inherent territory and the entry into the territorial waters by the Chinese government ships is absolutely unacceptable," a Japanese Foreign Ministry official said.

"On top of that, there appears to have been a flight of a drone. We lodged a stern protest against this unilateral escalation of the situation by China."

The Chinese embassy responded to the Japanese protest by reiterating "China's own stance" on the islands, the official added.

In a brief statement on its website, China's State Oceanic Administration confirmed that four coast guard vessels had been patrolling by the islands, but made no mention of any drone.

China routinely rejects Japanese criticism of such patrols, saying its ships have every right to operate in what China calls its territorial waters.