The territorial dispute over the Tokyo-controlled islets, known as the Senkaku by Japan and the Diaoyu by China, is a long-running sore point between the countries.

Relations deteriorated in 2012 when Tokyo "nationalised" some of these remote islands, and Japanese officials regularly protest at the presence of Chinese coast guard and other vessels in the surrounding waters.

On Monday, Japan's coast guard observed four China Maritime Police Bureau vessels sailing in the "contiguous" zone adjacent to Japan's territorial sea near the island chain.

Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters.

It was the 158th consecutive day that Chinese boats were spotted there -- surpassing the previous record of 157 days in 2021, top government spokesman Yoshimasa Hayashi said.

"The government considers this series of navigations within the contiguous zone and intrusions into territorial waters an extremely serious matter," he told reporters.

Hayashi said Prime Minister Fumio Kishida had expressed concern over the issue at a bilateral meeting with Chinese Premier Li Qiang in Seoul on Sunday.

Kishida is in the South Korean capital for the first trilateral summit with Li and President Yoon Suk Yeol in nearly five years. It was held on Monday morning.

In April, Beijing lodged a protest with Tokyo after a group of Japanese lawmakers visited the disputed isles, a trip the Chinese embassy denounced as "provocative".

After years of negotiations, the two countries have set up a military hotline to avert unexpected clashes in the East China Sea, with the first call held a year ago.