Japan's top government spokesman on Wednesday declared himself dumbfounded after a recent prime minister said he understood China's claim to islands at the centre of a bitter row between Tokyo and Beijing.

Yukio Hatoyama, whose brief term as premier was almost universally regarded as a flop, told Hong Kong-based Phoenix Television it was "unavoidable" that China believed Japan "stole" the Senkaku islands.

The uninhabited outcrop in the East China Sea, which Beijing claims as the Diaoyus, is the subject of a decades-old dispute that has flared badly in the last year. Both countries continue to send official ships to the area to press their ownership.

Japan says it brought previously unclaimed islands under its control in 1895. China says they were illegally snatched and should have been returned alongside other occupied territories after World War II.

During the interview, which was broadcast in China on Tuesday, Hatoyama said: "It is unavoidable that the Chinese side thinks Japan stole" the islands.

As a media storm gathered, he told Japanese reporters later in the day that he had meant to say there was "a possibility" that China might think that way.

Japan's chief government spokesman, Yoshihide Suga, said Hatoyama's comments were "outrageous".

"I was completely at a loss for words when I heard about his remarks. It was literally jaw-dropping," he told a news conference on Tuesday.

"It is outrageous and unforgivable that a former prime minister has said things that hurt our national interests," Suga said.

Asked again on Wednesday about the episode, Suga said he had still not recovered his tongue.

"I said yesterday that I was left open-mouthed. I still remain so, my mouth remains open," he said.

Despite the regular stand-off between official vessels at the islands, Japan's stated position is that there is no dispute over the archipelago.

Internet users in China offered an enthusiastic response to Hatoyama's comments.

"Yukio Hatoyama is an honest politician whereas (Prime Minister Shinzo) Abe is a lying politician, a swindler who denies there is any dispute over the Diaoyu islands," said Zouxiangshenlanhepingweishi on microblog Weibo.

Hatoyama, who led the now-opposition Democratic Party of Japan and served as prime minister between September 2009 and June 2010, has left politics, but as an heir to the Bridgestone tyre empire remains active in business circles.

His time in office was marked by confusion and policy flip-flops, including on the US military presence in Okinawa, which managed to alienate voters and irritate Washington without achieving any drawdown of troops.

Nicknamed "The Alien", in part because of his wide-eyed expression, Hatoyama's sometimes oddball comments proved a stumbling block for party managers.

He is a fourth-generation politician and the second in the family to become prime minister, after his grandfather. His other grandfather founded Bridgestone.

Hatoyama's wife Miyuki is a former actress who became a lifestyle guru with an interest in spirituality. She famously said her soul once visited Venus on a triangular spaceship and that she met Tom Cruise in a previous life.