Japanese coast guard ships searched Friday for eight missing Chinese crewmembers after rescuing six others from a fishing boat that collided with a cargo ship off disputed islands in the East China Sea, government officials said.

The boat apparently sank Thursday after colliding with the Greek-flagged ship off the Japan-controlled Senkaku islands, which China also claims. The collision occurred as hundreds of Chinese fishing boats have gathered around the islands since early August following the start of the mackerel season.

Up to 15 Chinese coast guard vessels — four to five times the usual number — were also in the area. Foreign Ministry officials say some have been converted from warships and are armed. The increase prompted Japan to lodge protests several times a day for a week.

Officials said some of the Chinese ships left following the accident, and the Chinese side thanked Japan for its rescue efforts.

Relations between Japan and China have long been strained over how to develop undersea gas deposits in the area, as well as by their wartime history, a sensitive topic during recent months because of end-of-World War II anniversaries.

Japanese officials have raised concerns over China's increasingly assertive activity around the Senkaku islands. Over the past few years, China has built a dozen gas development platforms in the East China Sea, and Japan has recently discovered that China has installed radar facilities and surveillance cameras there, they said.

Japanese media reports say the recent escalation in Chinese activity around the disputed islands may be seen as a warning against planned visits by members of Prime Minister Shinzo Abe's Cabinet to Tokyo's Yasukuni Shrine, which honors war criminals among Japan's war dead, for the Aug. 15 anniversary of the end of World War II.