Japan scrambled fighter jets on Thursday after four Chinese coastguard vessels entered what Japan considers its territorial waters near disputed East China Sea islets and a drone-like object flew near one ship, Japan said.

It was the first such flight near the islands witnessed by Japanese officials, although the incident took 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.

“This is escalating the situation and absolutely unacceptable,” Defence Minister Tomomi Inada told a news conference on Friday, referring to the incursion and drone flight.

“We regard this as a serious infringement of Japan's sovereignty.”

Ms. Inada said two F-15 fighter jets, one E-2C early warning aircraft and an AWACS surveillance plane were sent to the scene.

Kenji Kanasugi, director-general of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, lodged a stern protest with the Chinese embassy in Tokyo by telephone, a ministry official said.

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

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

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