Japan's government underestimated how China would react to its decision to buy islands at the centre of a bitter territorial dispute, Tokyo's former envoy to Beijing said Monday.

"I don't know why such a decision was made in a hurried manner and at such a time" Uichiro Niwa told a news conference, speaking of then-prime minister Yoshihiko Noda's September announcement he was buying three of the Senkakus.

Noda's government maintained the purchase was little more than administrative -- transferring uninhabited islands in the East China Sea, known as the Diaoyus in China, from a private Japanese citizen to the state.

The administration made no secret of the fact that it was intended to outfox an attempt to buy the islands by the nationalistic then-mayor of Tokyo Shintaro Ishihara, which it judged would be a worse provocation to Beijing.

"The government of Japan transferred ownership from an individual to the state based on its domestic law, but once an issue involves crossing waters, it becomes a diplomatic issue," Niwa told reporters.

"I think (Japan) should have taken it more seriously and offered a diplomatic explanation to China."

The businessman-turned-envoy said China's President Hu Jintao lost face when Noda announced the nationalisation only days after Hu warned against the move.

The two leaders held unofficial talks on the sidelines of last year's Asia-Pacific summit in Vladivostok, during which Hu reportedly told Noda to understand Beijing's seriousness and handle the case from a broad perspective.

"(Noda) made Hu Jintao lose face as head of state," which led to "raging reactions" from Beijing, Niwa said.

"China is a country that places a great deal of importance on saving face," he added. "The Japanese side appeared to have underestimated it to a certain degree."

The two countries have argued for decades about the ownership of the archipelago but the dispute flared anew after Ishihara announced his bid.

The nationalisation sparked large demonstrations in China and cooled the multi-billion dollar trade relationship as a consumer boycott weighing heavily on Japanese firms, including the country's auto and electronics giants.

Beijing has repeatedly sent its ships, and latterly its planes, to the area in a bid to assert its control over the chain.

Last week a Japanese emissary met China's incoming president Xi Jinping and handed him a letter from Japanese Prime Minister Shinzo Abe. The contents were not disclosed.