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BEIJING : Beijing said it had issued a sharp warning to Tokyo over media reports that Japan had positioned patrol vessels capable of carrying helicopters near a disputed island chain.

A senior Chinese foreign ministry official made "serious representations" to Japan's embassy in China over the reports that appeared in Japanese media, according to a statement posted on the ministry's website Wednesday.

The statement quoted the unnamed official as telling Japan Tuesday that if the reports of Japanese ships in the Diaoyu islands were true, it marked "a severe violation of Chinese sovereignty, and China expresses strong concern."

Chinese state media has said the ships were reportedly Japanese coastguard vessels carrying helicopters.

"If Japan persists in taking actions that escalate the Diaoyu problem, China will have no choice but to react strongly. The Japanese side must be clear on this," it said.

The archipelago in the East China Sea, known in Japan as the Senkaku 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.

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.