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 rumbling 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 says it has now signed a purchase contract.
“This should cause no problem for Japan’s ties with other countries and regions,” said Chief Cabinet Secretary Osamu Fujimura.
This is a row as much about politics and emotion as the strategic significance of the islands themselves. Domestic uncertainty in both countries plays a part too.
China has nonetheless called Japan’s move illegal and warned it would affect ties. State-run media have 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 news agency.
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.