Chinese state media says two government patrol ships have been sent to islands disputed with Japan, as Japan sealed a deal to purchase the islands.
The ships had reached waters near the islands - known as Senkaku in Japan and Diaoyu in China - to "assert the country's sovereignty", Xinhua said.
Japan confirmed on Tuesday it had signed a contract to buy three of the islands from their private owner.
Tension has been brewing between the two countries over the East China Sea.
Japan controls the uninhabited but resource-rich islands, which are also claimed by Taiwan. Some had been in the hands of a private Japanese owner but the government confirmed on Tuesday that it had signed a purchase contract.
"This should cause no problem for Japan's ties with other countries and regions," said Chief Cabinet Secretary Osamu Fujimura.
"We have absolutely no desire for any repercussions as far as Japan-China relations are concerned. It is important that we avoid misunderstanding and unforeseen problems."
Japan said on Monday that it was buying the islands to promote their stable and peaceful management - but China called the move illegal and warned it would affect ties.
State-run media has carried strongly worded statements on the issue.
"The Chinese government will not sit idly by watching its territorial sovereignty being infringed upon," the Foreign Ministry said in a statement issued on Monday and carried by Xinhua.
"Should the Japanese side insist on going its own way, it shall have to bear all serious consequences arising therefrom."
Chinese Premier Wen Jiabao also reiterated China's stand on Monday.
"The Diaoyu islands are an inalienable part of China's territory, and the Chinese government and its people will absolutely make no concession on issues concerning its sovereignty and territorial integrity," he said.
The announcement of the dispatch of the patrol boats came in a brief Xinhua report.
China Marine Surveillance - a maritime law enforcement agency - had "drafted an action plan for safeguarding the sovereignty and would take actions pending the development of the situation", it said.
The islands, which lie south of Okinawa and north of Taiwan, sit in key shipping lanes and are thought to lie close to gas deposits.