Chinese state-owned ships entered the territorial waters of disputed islands on Friday, Japan's coastguard said, in the first intrusion since a new government was elected in Tokyo.
The move is a setback to hopes in Japan that Beijing might use the poll as a chance for a fresh start after months of bitter wrangling and rhetoric over an issue that neither side is prepared to budge on.
"Three Chinese surveillance ships entered the territorial waters near Kubajima," said a Japanese coastguard official, referring to one of the islands in the Senkaku chain, known as Diaoyu in China.
The coastguard said three Chinese ships were spotted northwest of Kubajima island at around 10:20 am (0120 GMT).
A fisheries patrol ship was also in contiguous waters 37 kilometres (23 miles) west-northwest of Uotsurijima island, it said.
China has sent its official ships into the islands' waters 19 times since Tokyo nationalised the chain in September, the coastguard said, with analysts saying Beijing intends to prove it can come and go as it pleases in the area.
The ante was upped last week when a Chinese plane overflew the area, in what Japan said was the first time Beijing had breached its airspace since at least 1958. Tokyo scrambled fighter jets in response.
But the State Oceanic Administration vessels have remained outside the 12-nautical-mile ring of the archipelago's territorial waters since Sunday's election, in which the hawkish Shinzo Abe swept to power, vowing a tough line on Beijing.
In one of his first broadcast interviews after the poll win he said there was no room for compromise in the row and put the onus for improved relations on Beijing.
"Japan and China need to share the recognition that having good relations is in the national interests of both countries," he said. "China lacks this recognition a little bit. I want them to think anew about mutually beneficial strategic relations."
Abe has pushed an agenda that includes upgrading the country's "Self Defence Forces" to make them a full-scale military, and has spoken of wanting to revise Japan's pacifist constitution.
But analysts have said at least some of this could be posturing.
They point to the pragmatism of his earlier 2006-2007 tenure as prime minister, when his opinions on controversial issues that could aggravate China were ambiguous or were just left unsaid.
As premier he stayed away from Yasukuni Shrine, which honours Japan's war dead, including Class A war criminals, and is a running sore in Tokyo's relations with its neighbours.
Abe also made China his first foreign destination.
Following his victory on Sunday, he said he would make rebuilding Japan's alliance with Washington his top foreign policy goal and said it would be the first place he visits after assuming office.
Despite warm words about the importance of economic ties with Beijing -- China is Japan's biggest trading partner -- Abe stressed the need to build relations with other countries, such as India and Australia.