A collision between a Chinese fishing boat and a Greece-flagged merchant ship near the Senkaku Islands in the East China Sea early Thursday led to a rescue operation by the Japan Coast Guard and a note of thanks from Beijing.

The coast guard rescued six Chinese crew members and was searching for eight missing, the agency said, after the fishing boat sank.

The boat collided with the 300-meter-long bulk carrier Anangel Courage about 65 km off Uotsuri Island, the largest in the Senkaku group of islets, just after 5 a.m.

The cargo ship issued a distress signal, and the coast guard dispatched a vessel and a plane to the area, the agency said. Weather conditions in the area were poor at the time.

Tokyo informed Beijing of the incident and the Chinese side “expressed appreciation” for the operation, the Foreign Ministry said in a statement.

Hundreds of Chinese boats are currently fishing in waters close to the disputed islands, the ministry has said, with multiple Chinese patrol vessels maneuvering among them. At least some of the government ships appear to be armed.

The Chinese ambassador in Tokyo said Wednesday that the flotilla is a response to heavy potential fish catches in the area, according to an official from Japan’s ruling party.

Toshihiro Nikai, secretary-general of the Liberal Democratic Party, quoted Ambassador Cheng Yonghua as saying “fish were markedly concentrated” around the islets.

China says the Japan-controlled Senkakus are Chinese territory. It knows the islets as Diaoyu.

On Wednesday, the Foreign Ministry issued a nine-page dossier detailing alleged intrusions with photographs of some of the vessels and lists of protests submitted to the Chinese side.

“There are approximately 200 to 300 fishing vessels operating in the contiguous zone of the Senkaku Islands, and a large number of Chinese government vessels,” the report said.

It said up to 16 patrol ships have been in the area, distributing images of the ships and identifying them as hailing from three government agencies: China Coast Guard, China Marine Surveillance and China’s Fisheries Law Enforcement Command.

The dossier listed examples of protests Tokyo has submitted to Beijing since Aug. 5. On Sunday alone, officials filed 10 separate protests with Chinese diplomats in Tokyo and the Ministry of Foreign Affairs in Beijing.

“Despite Japan repeatedly lodging strong protests with the Chinese side, the Chinese side has continued to take unilateral actions that raise tensions on the ground, and this is absolutely unacceptable,” the Foreign Ministry said in the document.

On Wednesday, the U.S. State Department restated that the Senkaku Islands fall under Washington’s treaty of mutual defense with Tokyo.

“We are in close communication with the Japanese as allies and are also concerned about the increase of Chinese coast guard vessels in the vicinity of the islands,” State Department Spokeswoman Elizabeth Trudeau said.

“We oppose any unilateral action that seeks to undermine Japan’s administration of the Senkaku Islands.”

Foreign Minister Fumio Kisihida on Tuesday said while relations with Beijing had deteriorated markedly, the door for dialogue remained open.

Information from Kyodo added