China has sent two surveillance ships to a group of islands at the center of a territorial dispute with Japan, even as Tokyo finalized a deal to purchase them from their private landowners.

The official Xinhua news agency said the China Marine Surveillance ships arrived Tuesday at the waters surrounding the islands “to assert the country's sovereignty.”

Meanwhile, Japanese media said Tuesday that Tokyo has signed a contract to purchase three of the islands from their private Japanese owner for $26 million.

The largely uninhabited islets, known in China as Diaoyu and in Japan as Senkaku, lie near strategic shipping and fishing grounds that are also thought to hold oil and gas deposits.

Japan announced the deal Monday, drawing immediate condemnation from Beijing's foreign ministry, which said it would not “sit back and watch its territorial sovereignty violated.” Beijing also summoned the Japanese ambassador to protest the plan.

In announcing the move Monday, Japanese Cabinet secretary Osamu Fujimura said the decision to buy the islands from their private Japanese owners was not meant to anger China, but to create a “stable and secure environment.”

The islands have a long history of stoking nationalist tensions and damaging relations between Asia's two largest economies. But the dispute heated up in recent months as Japanese and Chinese nationalists sailed to the islands and swam ashore to assert their countries' claims, prompting the two governments to accuse each other of provocation.

Some observers say that Japan's purchase of the island was partly prompted by Tokyo's outspoken nationalist governor Shintaro Ishihara, who threatened earlier this year to purchase the islands because he said Japan was not doing enough to protect them from Chinese claims.