Chinese ships were spotted near a group of the Japan-controlled Senkaku Islands in the East China Sea for the 100th straight day on Wednesday, the longest streak since Tokyo put them under state control in 2012.

Japan said China's repeated dispatch of vessels to the Senkakus is an "extremely serious" issue, prompting Tokyo to beef up patrols and lodge protests with Beijing.

File photo taken in September 2012 shows the Japan-controlled Senkaku Islands in the East China Sea. Taiwan and China claim the uninhabited islands as their own. (Kyodo)

In Beijing, Foreign Ministry spokesman Wang Wenbin told reporters later in the day that China does not accept such protests by Japan as the islets have been its "inherent territory since ancient times."

China has an "inherent right to carry out patrol and law enforcement near the waters of the Diaoyu Islands," Wang said, using the country's name for the isles.

During the 100 days from April 14, Chinese ships were spotted in Japan's territorial waters around the uninhabited islands on 11 days, according to Japanese officials.

"The repeated activities are extremely serious. Japan Coast Guard patrol ships have issued warnings, and we have protested to the Chinese side through diplomatic channels over and over again," Chief Cabinet Secretary Yoshihide Suga told a press conference.

"We will respond to the Chinese side calmly based on our resolute stance," the top government spokesman said, adding that Tokyo is determined to defend its territory.

The Japan Coast Guard said four Chinese Coast Guard ships were spotted in waters around the Senkakus on Wednesday, with one of them equipped with what looked to be a machine gun. Coast guard patrol boats warned the four vessels against entering the territorial waters.

China started to lay claim to the tiny islands, located some 400 kilometers west of Okinawa in southwestern Japan, in the 1970s after expectations grew for abundant oil and gas deposits in the area.

Bilateral tensions over the islets spiked after Japan put them under state control in September 2012, setting the stage for years of frayed diplomatic ties.

In recent years, Japan-China relations have thawed against the backdrop of rivalry between Beijing and Washington. A state visit by Chinese President Xi Jinping was due to take place this spring until the coronavirus pandemic led to its postponement.

Still, China's frequent dispatch of ships to waters around the islands has sparked calls from Japanese conservative lawmakers to cancel the plan to receive Xi as a state guest, at a time when Beijing's tightening of its grip on semiautonomous Hong Kong has drawn international criticism.

Some of the Chinese ships sent to waters around the Senkakus have been spotted tracking or trying to track Japanese fishing boats operating there.

In early July, two Chinese Coast Guard ships stayed in Japanese territorial waters around the islands for over 39 hours, the longest period since September 2012.

Japan has also urged China to stop marine research activities without permission after a Chinese vessel with surveying equipment in the sea was spotted earlier this month in Japan's exclusive economic zone near Okinotori Island.

Japan uses the island, its southernmost territory, as its demarcation point for a 200-nautical-mile EEZ, but China calls them rocks.