Japanese Prime Minister Taro Aso said Thursday Tokyo would call on the US to help it defend a disputed island chain, triggering immediate protest from China.

"The Senkaku islands are part of Japan's sovereign territory," said Aso at a parliament panel.

"Therefore, they should be protected under the Japan-US security pact" if they are to be invaded by another country, he added.

The archipelago in the East China Sea, known in China as the Diaoyu islands, has long been a bone of contention between the two Asian giants, both of whom claim rights to lucrative undersea gas fields in the area.

Pressed by an opposition member at parliament, Aso said his government would "reconfirm at an early date" the security deal with the United States over the islands.

His remarks prompted an immediate protest from Beijing.

"China expresses strong disatisfaction" with Aso's comments, said China's foreign ministry spokesman Ma Zhaoxu, adding Beijing "has already lodged stern representations with the Japanese side".

"The Diaoyu islands … have been part of Chinese territory for a long time, and China has indisputable sovereignty over these," Ma said in a statement on its website.

Japan and China, two of the world's biggest energy importers, struck a deal last June to end the energy dispute by jointly developing one of them and holding talks on the others.

But Japan recently protested that China was unilaterally developing the Tianwaitian field and both sides have continued to trade accusations of encroachment in the islands by each other's vessels.

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