Three Chinese government ships were in Japanese territorial waters off a disputed island chain on Monday, the coastguard said, in the latest salvo of an increasingly heated international dispute.
The move came a day after China dealt a diplomatic snub to Japan by postponing long-planned events marking the 40th anniversary of ties, as relations plumb depths not seen for decades.
It also came as Japanese Prime Minister Yoshihiko Noda warned Beijing's uncompromising stance could affect its economy and have knock-on effects on the wider world.
Japan's coastguard said that as of 0200 GMT, two maritime surveillance ships and one fishery patrol boat were in sovereign waters off Uotsurijima, the largest island in the Japanese-administered Senkaku chain, which China claims as the Diaoyus.
The ships are not naval vessels; maritime surveillance comes under the State Oceanic Bureau, which is part of the Ministry of Land and Resources. Their roles include law enforcement in Chinese waters.
Fisheries patrol boats come under the aegis of China's Agriculture Ministry, and are responsible for policing fishing and marine resources.
The coastguard said six other vessels were in contiguous waters, an area under international law that extends up to 12 nautical miles outside a territory.
Osamu Fujimura, Japan's top government spokesman and chief cabinet secretary, said Japan has "protested strongly" over the intrusion through diplomatic channels.
Up to 14 Chinese government ships have been in the area for over a week, dipping in and out of contiguous waters.
Beijing sent vessels to the islands on September 11, the day Tokyo announced it had completed a deal to buy three of the uninhabited rocks from their private owner.
Commentators say the nationalisation of the islands was intended to prevent their purchase by the nationalist governor of Tokyo, who said he wanted to develop them.
But Beijing reacted angrily and unleashed a firestorm of protest, which also saw sometimes violent rallies rocking several cities, with Japanese businesses suffering vandalism and arson at the hands of rioters.
On Sunday, Chinese state media announced Beijing was "postponing" celebrations to mark the 40th anniversary of the normalisation of diplomatic ties.
A ceremony, which was due to take place on Thursday, was to be hosted by a friendship organisation. It has been held every decade and never before been cancelled.
The Japanese government on Monday described the cancellation as "regrettable".
"It is important that we deepen the overall strategic and mutually beneficial relationship between Japan and China, without letting an individual event affect ties," Fujimura told reporters.
Asia's two largest economies have wrangled since the 1970s about the islands, which lie on important shipping lanes and are believed to harbour mineral resources.
Periodically the row flares, sometimes affecting the multi-billion dollar trade ties between the two nations.
Disputes have usually been salved with the passage of time.
But the latest episode, which comes as China is in the process of a delicate leadership transition and as Japan's political scene has become increasingly unstable, shows no signs of dying down.
On Saturday around 800 Japanese demonstrators waved national flags as they marched through downtown Tokyo, denouncing Beijing as a "brute state" and "fascist" in the first mass-rally since the dispute began.
Japanese dailies the Mainichi and the Yomiuri said Monday that Tokyo was hoping the two countries' foreign ministers would be able to meet on the sidelines of the UN General Assembly in New York this week.
In an interview with the Wall Street Journal ahead of the UN meet, Noda warned China's attitude could damage its economy.
He said Japanese companies were now facing a form of economic harassment in China.
"Recent delays in customs and visa issuance are of concern," he said.
"Damaging our ties over such things would be bad for not just the two countries' economies, but for the global economy."