TOKYO - Eleven Chinese government ships entered waters around Japanese-administered islands Tuesday, Japan's coastguard said, hours after two Japanese activists swam to one of the disputed islets.
None of the vessels had entered territorial waters, but all had gone into an area known under international maritime law as the contiguous zone, in a move that came as fresh anti-Japan protests rocked Chinese cities.
"As of 4:30 pm (0730 GMT), 10 Chinese maritime surveillance ships were spotted in contiguous waters off Uotsurijima," a coastguard spokesman told AFP, referring to the largest island in the Senkakus, a chain China calls Diaoyu.
"A fisheries patrol boat re-entered contiguous waters off Kubajima at 12:16 pm but we have not been informed of its location since."
Earlier in the day, Japan's top government spokesman said two Japanese people had swum to Uotsurijima.
"Two Japanese landed on Uotsurijima at about 9:30 am (0030 GMT)," Chief Cabinet Secretary Osamu Fujimura told reporters. "The coastguard said they have already left," he said.
Jiji Press, citing police in Okinawa, said the two were from Japan's main southern island of Kyushu.
They arrived in the area in a small boat and swam to the island, a spokesman at the coastguard in Okinawa said, adding they were back in their boat shortly afterwards.
The landing was the fourth by Japanese this year and came weeks after seven pro-Beijing activists made it ashore on the same island, marking a sharp downturn in relations between two of the world's biggest economies.
Earlier in the day, Japan's coastguard said it had warned away a fisheries patrol vessel that had been sighted 42 kilometres (26 miles) north-northwest of Uotsurijima at 7:00 am (2200 GMT Monday).
The Chinese vessel later moved close to another island, Kubajima, in the chain.
"Our patrol vessels are warning it not to enter our country's territorial waters by radio and other means," the coastguard said in a statement.
The ship had told Japanese vessels that it was "carrying out legitimate activity", arguing the islands are Chinese sovereign territory.
Widespread anti-Japanese protests, some of them violent, have erupted acrossChina in recent days over the East China Sea islands, which lie in rich fishing waters and on important shipping lanes.
Major Japanese firms, including Canon and Honda have suspended operations at several plants in China, according to officials and reports Monday.
After meetings in Tokyo with senior Japanese officials on Monday, US Defense Secretary Leon Panetta -- who later travelled on to Beijing -- urged "calm and restraint on all sides".
A fresh wave of anti-Japan rallies was in progress Tuesday, the anniversary of the 1931 "Mukden incident" that led to Japan's invasion of Manchuria, which is commemorated every year in China.
China and Japan have close economic and business ties, with two-way trade totalling $342.9 billion last year, according to Chinese figures.
But the two countries' political relationship is often tense due to the territorial dispute and Chinese resentment over past conflicts and atrocities.
A landing on the island by pro-Beijing nationalists in August marked the start of a sudden worsening of relations between China and Japan.
Tokyo announced last week it had bought three of the islands, which it administers, from their private Japanese landowner.
A Taiwanese politician said Tuesday a group of Taiwanese fishermen were planning to sail this week to the archipelago.
About 60 fishing boats each carrying five to six people are expected to head for the islands Saturday from a port in northeast Taiwan's Ilan county, said Lin Chi-shan, a co-organiser of the event and a member of Ilan county council.
Taiwan also claims the islands, which are uninhabited but strategically important.