A smouldering islet created by undersea volcanic eruptions off Japan's Pacific coast has melded to a nearby island, the Japanese coastguard said Thursday.

Officials overflying the new landmass said it had merged at two points with Nishino-shima, an uninhabited volcanic island in the Ogasawara (Bonin) chain, some 1,000 kilometres (620 miles) south of Tokyo.

Two craters on the islet have been erupting "at an interval of 30 seconds to one minute," spewing brown smoke about 100 metres (330 feet) high, a coastguard statement said.

Pale volcanic gas and ash-grey smoke are also oozing out.

The islet was first spotted on November 20, some 200 metres from Nishino-shima, which is estimated to be 10 million years old.

It is now sized about 450 metres from east to west and 500 metres from north to south, with ground space measuring about 0.15 square kilometres (0.06 square miles), the statement said.

"The amount of volcanic smoke has apparently grown with a continuous flow of lava," Tokyo Institute of Technology geology professor Kenji Nogami reported from the plane.

"The supply of magma from the deep subsurface is presumably increasing."

Similar eruptions in the early 1970s and mid-80s created tiny islets in Japan's territory that have since been partially or completely eaten up by the ocean.

While the new formation is in uncontested waters, its emergence comes as Tokyo is embroiled in a row with Beijing over the sovereignty of a small archipelago in the East China Sea. The sudden appearance sparked quips from ministers about the expansion of Japan's territorial waters.