Japan's government has agreed to buy islands at the centre of a territorial row with China, reports said Wednesday, as it tries to both placate nationalists and prevent ties with Beijing deteriorating further.

Prime Minister Yoshihiko Noda has been pushed into the deal after a canny move by the right-wing governor of Tokyo who said he wanted to purchase them to protect them from Chinese claims of ownership.

Beijing's response to the reports was muted, with a foreign ministry spokesman saying China was monitoring the situation and would "take necessary measures to defend its national territorial sovereignty".

Tokyo will pay private Japanese landowners 2.05 billion yen ($26 million) for three of the islands, known as Senkaku in Japan and Diaoyu in China, the Yomiuri Shimbun and Kyodo News reported, citing unnamed government sources.

Deputy Chief Cabinet secretary Hiroyuki Nagahama met the landowners on Monday and struck the deal, which includes Uotsurijima, the largest in the chain, both outlets said.

Contracts are expected to be signed next week, the Nikkei newspaper said.

At a news conference Chief Cabinet Secretary Osamu Fujimura refused to confirm the reports, but said negotiations were under way.

"We are exchanging views with the landowners in various ways, but that process is ongoing," he said.

"We cannot comment on the contents at all. As a government, we will make a firm announcement after procedures are appropriately completed."

Noda plans to formally tell the Chinese about the purchase on the sidelines of the UN assembly later this month, the Asahi Shimbun said, but a Japan-China summit has not yet been set.

Four of the islands in the remote, but strategically coveted archipelago, are owned by the Kurihara family, who bought them in the 1970s and 80s. The government already owns the fifth.

The original Japanese owner had established factories processing bonito fish and albatross feathers on one of the islands, which were abandoned during World War II and then came under US control until 1972.

The government stepped in with its bid after Tokyo's outspoken nationalist governor, Shintaro Ishihara, took all sides by surprise and announced his intention to buy the archipelago for the metropolitan government.

He charges that the national government, which already leases the four islands it does not own, has not done enough to protect Japanese territory from threats by China.

Ishihara, who gathered millions of dollars of donations towards his purchase plan, had said he wanted to develop the islands and at the weekend sent a team of surveyors to nearby waters.

The government has no plans for any construction on the archipelago in an effort to avoid further friction with China and Taiwan, which also claims the islands, the Asahi said.

Often testy Japan-China ties took a turn for the worse in August when pro-Beijing activists landed on Uotsurijima. They were arrested by Japanese authorities and deported.

Days later about a dozen Japanese nationalists raised their country's flag on the same island, prompting protests in cities across China.

Last week the Japanese ambassador's car was targeted in Beijing when an unidentified man ripped the national flag off the vehicle.

Fujimura told reporters on Wednesday that Japan -- which insists there is no territorial dispute over the islands -- was working towards better ties with Beijing.

"It is true that there are some problems from time to time, but we regard it as a neighbour country with which we will further strengthen our strategic partnership."

Yoshinobu Yamamoto, honorary professor of international politics at the University of Tokyo, said the government's purchase of the Senkakus would make little material difference.

"Practically speaking, there won't be any change in sovereignty if the islands are owned by private individuals, Tokyo or the government," he said. "This is a gesture to display Japan's effective control."

Yamamoto said Japan was stuck between nationalist pressure and an increasingly assertive China.

"Domestically, the decision was obviously caused by Governor Ishihara's move," he said.

"I don't think Japan can find a solution to the territorial dispute. All it can do is to maintain the status quo without enlarging the problem."

The chain, 2,000 kilometres (1,250 miles) from Tokyo, but less than 200 kilometres from Taiwan, lies on vital shipping lanes, and is believed to be near potentially rich gas fields.