Japan's coast guard has captured on camera the emergence of a new island from an erupting undersea volcano off the southern Ogasawara chain.

It's just 200 metres in diameter but the powerful eruption sent plumes of smoke almost 600 metres into the air.

Japan's government spokesman welcomed the arrival of the new territory but said it was too early to consider naming it.

If the new arrival survives it'll join Japan's estimated 6, 000 islands.

It would technically be part of Tokyo even though it's more than 1, 000 kilometres away.

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